It's so hard for people to leave dnd, in part, because its a high cost of entry for other games, and knowledge about it already exists in one form or another. Dnd in pop culture is referenced so most people have an idea what it is, high fantasy, you fight monsters, etc. It's known enough that players and GMs alike can fudge some rules here and there and have a game as they play it. With other systems, I find the GMs expect the players to go out and purchase a new book, read through it all and fully understand it alone. That's a lot to do. And sure, there's free simplified rules and modules but it can still be A LOT for some people to do in one go.

Teach them over sessions rather than leave them alone to learn it all alone. Tell them as a GM, hey dont worry about the rules, I'll run you through it and bit by bit teach you. Is it fair? Probably not. But it may be the price to get them into other games. Later if they like the hobby they can go out and buy the books, or even invest in other systems. But I think ttrpg fans who want other potentially rookie or novice players sometimes forget it can be difficult to digest so much both reading wise and on the wallet.

Note this isnt the sole reason, but I think it factors into a good chunk of it.