I pretty much never play traditional RPGs these days (with the exception of old-school/OSR-style play, which is lots of fun, but very different in both structural and subtle ways), so I have to speak of this a little bit as an outsider.
However, I think there are a number of reasons these games appeal to people that go beyond circumstances (many, maybe even most, gamers seem to be entirely unaware that RPGs aside from D&D even exist!), social and brand reinforcement (it’s the popular game! easy to find players!), and familiarity and nostalgia. Those seem to have been covered pretty thoroughly already in this thread, after all!
For instance, the very familiar GM/player dynamic (especially in games with Rule Zero heavily in effect) means that you can effortlessly go from game to game without needing to learn new skills or adjust to a different playstyle. Many traditional RPGs are basically the same game, with different stats and different dice to roll, and that makes any skills and habits you already have very transferable. That’s a thing of convenience (after all, many people simply don’t like learning new tricks).
For many people, the ability to explore a well-defined system with lots of rules, character options, and things to expect/built towards is a huge draw. This kind of player might get tired of Lady Blackbird after five sessions, but in their D&D campaign, there are so many classes to “try out”, and you’re always leveling up, getting new abilities, and fighting new monsters. This builds and maintains long-term investment and long-term interest, which is important for anyone looking to run a long, continuous campaign. (I’ve noticed the “indie” games which feature more in-depth material in terms of system to explore - like Burning Wheel, Blades in the Dark, some PbtA games, etc - seem to “catch on” more widely or more easily than rather bare-bones or “rules-light” games in the same design style and genre. So I think this is pretty important to a lot of people.)
If you’re looking to put together a truly long-term game, having a system set out which paces you through the development of a game over the long term the way that, say, D&D’s level progression does, is a really valuable tool. Consider how all of D&D’s mechanics and the overall design both forces you and helps you to stretch out a long, epic, zero-to-hero format for your game, and how much more difficult that would be to do without that kind of support! (There’s no reason, of course, why a game designed in a totally different style couldn’t do the same thing, but it’s not really a design trend we’ve seen just yet.)
Some people really like being able to lean on published products: if you’re not super keen on developing your own material, being able to buy a campaign setting and a book of magical items and a book of special spells and then a constant supply of adventure modules is a huge benefit. This might be because you’re not interested in the creative process involved in this kind of activity (I don’t really buy the argument that learning published material is faster than writing your own, but some people say so, as well), because you really like/enjoy the material a certain publisher produces (whether because you’re simply a fan of a certain writer, maybe, or because it’s better than what you’d come up with by yourself/selves), or because you like being able to lean on material someone else has created because it allows you to GM a game in a particular way (e.g. disclaiming authority over the contents and thereby acting like a impartial referee is something you want and need support for).
If your group is very much organized around a trusted, charismatic, auteur-like GM-figure, with the players used to showing up at the table and being entertained, a “trad” game can enable that really well. Switching to a more “indie” game, which may have rules or a play culture aimed at a more democratic distribution of creative powers, could actually remove some of that fun (and even harm the overall quality of the game) for such a group. I’ve seen such groups try a more “modern” or “non-traditional” design and feel really relieved when they go back to their regular game. Their GM can do an incredible job creating and sustaining an “experience” for the group, and trying games which mess with the distribution of creative authority just gets in the way of what they’re trying to do.
On a related note, more traditional-style RPGs are incredibly robust or flexible in terms of who shows up to play. You can handle a variety of players with a wide range of play styles, skills, experience, and engagement. Your D&D game will work just fine if Bob from the office drops in and joins the party for one session, in other words. Same goes for the number of players at the table: adding a fifth or sixth (or even a seventh!) player to the D&D table doesn’t change the fundamental dynamics too much, but some of my favourite games only really play well in a very narrow range (like how Fiasco is designed for 3-5 people, but really works best with 4).
This recently came up with a group I sometimes join. They tend to play in an “OSR style”, and it’s no problem for strangers or newbies to show up on any given week. Experienced, active, knowledgeable players can play alongside someone who is tired, stressed, unfamiliar with the game, and just wants to kind of sit and listen. The number of players showing up doesn’t matter. In comparison, more focused, narrative games I like to play are quite sensitive to the number of players, their skill/commitment level, and can really suffer if there is even just one person at the table who “doesn’t get it” or isn’t into the game.
In terms of how it “feels” to play, there is a certain dynamic tension present in games which have rules which more or less run as “the physics of the game world” and character death is always on the table. It’s a very different kind of experience than you get playing a character in, say, Fiasco, or, really, any game which guarantees you continued creative input and more or less guarantees that no matter what we choose to do, we’ll find the fun in it. When all you have are your character’s abilities and skills, and death is constantly on the line, there’s a different kind of energy at the table and a different level of tension in the fiction, as well.
Finally, some people simply don’t like or enjoy authoring fictional material, coming up with creative ideas, storytelling, and so on. We can speculate for a long time about why and whether it’s something that you can grow out of, but it’s a common position for some gamers to take. They don’t like the social and creative scrutiny that comes with expressing yourself in a certain way at the table, and enjoy a more “laid-back” game much more.
This can take pretty particular and specific forms. Maybe Julia doesn’t enjoy “acting out” her character and doing “voices” for them, but Julian has trouble switching stances and therefore struggles with games which ask him to make “author-level” decisions for his character. Their friend Xiao has trouble enjoying a sense of immersion when the mechanics are too math-heavy, but Jorge and Anisa just can’t get into a game that asks them to occasionally make decisions against their character’s interests. We all have “technical” preferences about how to engage with the hobby. I strive to be as flexible and diverse as I can when I play, but I don’t expect everyone else to do the same.
There are lots of different reasons some people enjoy “trad roleplaying”, and for any given person it could any combination of the above factors, and possibly many others.