I have been thinking this for years. I've never understood why so many gamers will only play 5e OGL systems (or maybe Pathfinder). The reason I most commonly hear is that gaming groups either cannot learn too many systems (eg, not enough time money to learn) or simply refuse to because it's what their group enjoys. But maybe to a lesser degree, some gamers simply don't know there are other TTRPG's out there?
With regards to thinking that groups can only play one or 2 systems, I find that dubious, and I wonder if these groups have even tried? As a pre-teen and teen in the 80s, using just the allowances of our gaming group, we bought and played so many games it would probably make people's heads here spin. I could probably name close to 30 game systems we played in a 6 year time span (I actually just started listing them down...and so far have 32 games I could remember off the top of my head). Granted, a few of those games we may have only played 1-4 sessions, but at least we gave it a try. Could we afford every supplement and module? No, but that's what imagination is for anyhow.
If the problem isn't money or lack of time, I suppose it could just be an unwillingness to try out anything else. But that's also just anathema to me. It'd be like eating pizza everyday for the rest of your life. Sure, pizza is great and switching the toppings around might be a small change, but I'd get sick to death of pizza too if that's all I ever ate.
As for gamers who aren't aware there are other TTRPG's beyond 5e OGL (or even D&D), I wouldn't have thought it possible in this day and age with the internet. But I was just watching a Youtube video from The Gaming Group where they listed 29 fantasy games that didn't use the OGL. Many comments were along the lines of "thank you so much...I wasn't aware these other games existed!". Back in the pre-internet days, I was lucky. We played at a hobby store that sold lots of RC cars, train sets, plastic models, and a decent amount of wargames and TTRPGs. In those days, I only remember a handful of stores in my state that were dedicated to TTRPGs, Board Games, or Miniature games (CCG's weren't really a thing until the early 90s with MtG). For example The Compleat Strategist and Enterprise 1701 (now Sci Fi City). These days, if there isn't a dedicated game store, they seem to combine with comic and collectible card stores. But in the age of the internet, I honestly scratch my head how people aren't aware of the vast universe of TTRPGs out there.
It also makes me wonder where the majority of people play now? At a store? At a member's house? On VTTs? If it's not at a store, then I can somewhat see why people may not be aware of other games, but it's still surprising to me.