Well, since this discussion is going on in another thread, thought it might be interesting to do a poll. Pick as many as apply to your group or players, feel free to add your reasons if they aren't captured!
Limited time is biggest factor. While we do venture occasionally into something new, they are on the very light side of things ( Cairn, Knave, Mork Borg). At this point, 2 systems that we use most are 5e and Chronicles of Darkness. Those are grandfathered in since we learned them back when we had more time on our hands and to be honest, for games we like to play, they just do their job.
Sunk cost fallacy, mostly. People start with a heavy game like D&D5E and think that all other games work like that so the idea of dropping $150 or more to switch systems and learn a new game they might not play is just a no-go. So, better to stick with the game they've already spent $150 or more on and deal with whatever quirks they're already used to instead of whatever quirks them might not like even more than what 5E has.
General resistance to change. I've heard them refer to all systems other than D&D 5e as "inferior" (despite not giving them a chance) and, specifically, indie games as "bad" (again, despite never having given them a chance). I die a little inside every time I hear this BS attitude.
I'm the forever-GM (which I love). So I tend to not be very democratic. Whatever I have interest in running is what I'm running. I don't ask them, I offer them. "Hey, I'm starting a new Symbaroum campaign. Are you in?" If they're not interested, they're not playing. I'm blessed by having a great availability of players.
That being said, I generally pitch them the game and my intentions. The reasons why they tend to be disinterested and get to skip their turn is generally just because they don't get excited by the fantasy I'm pitching. I might pitch Symbaroum and they go "Meh, I don't feel like doing dark fantasy."
Sometimes I'll get resistance because they enjoyed the last system or campaign so much that they want more of it. That's a good problem to have.
Usually when a game gets rejected locally, its either because of genre (the surviving group isn't a big fan of horror per se), or extremely specific mechanical elements (the reason Cypher is pretty much off the table).
We spent a good amount of time trying some other game systems over the last year and a half and I did see some in our group bounce off certain games that were too rules light and now if they get a whiff of narrative, rules light games, they tend to turn their noses up.
I find it mostly time. I get to play once a week for a few hours and we are playing 5e right now. We have a campaign going that advances slowly enough with the 1/week frame. Throw in a few missed weeks for whatever and then I really want to play the next week, and not skip 5e to play another game this week to get back to the campaign in 5e maybe next week.
Most of the players I've gamed with aren't going to spend money on new systems, even if they really like the game. Usually, I buy the game and spoon-feed them the ruleset. I've accepted that if I want to play/run anything besides DnD, I have to do all or most of the legwork.
Limited time to play. Learning a new system cuts into that time, and most powerful of all, we like 5e and it works so why change. Now, as DM I can probably ultimately put my foot down eventually and say, "I am tired of 5e; we are playing something else. Here are the options." It helps that if I financially boot the options they are more likely to be on board.