So I'm trying to think of a way to say this nicely. This is my personal feeling, and is flavored by past encounters with some absolutely horrible gamers, so try not to take it too personally.
While there are plausible excuses trotted out to stick to older versions of a game, like having spent time on rules retention or invested a great deal of money (sunken cost fallacy), I think that some people prefer older versions of games because it's more exclusionary.
It's HARD to calculate THAC0 with its "AC goes from 10 to 0, with 0 best, and some ACs are negative numbers, which are better than that, and your character has a number to hit Armor Class 0 (called THAC0), and if the number is positive you subtract it from your THAC0 to see what you need to hit, and if it's negative you add it to your THAC0 to see what you need to hit..." Compared to 3rd version and beyond's "Roll the dice and add your bonus to it, if it's equal or over the AC you hit," there's no contest. 3e is just better. Quicker in combat scenarios, easier to use, and even an average 10 year old can understand it.
But sticking with the older system makes that person feel superior for being able to 'get it' where other people just get bored or confused, and when newer versions come out which don't have all the complicated and contradictory rules that superior feeling is threatened.
I don't game to feel better than other people. I game to play with other people, and that makes me feel better, period.