That is, players getting so used to maps or game mechanics that they just keep playing the same maps over and over (hello A.T.C.S.) and reject every change to the game logic.
I think there are a few primary factors, a difficult learning curve, a lot of time invested in learning every nook and cranny (including the bugs as if they are features) of something that hasn’t had a significant change in awhile, a preference to not give up what is already known as a cost for something new. There are other causes to the symptoms you are describing besides invested skill.
For example other reasons that atcs is played so much is due to how the 1.1 vanilla game play’s issues can become more pronounced on most other maps, the map selection on the old 1.1 servers are very easily abusable, it is assumed that atcs is more preferable than it actually is and becomes a self fulfilling prophecy.
An example of another reason that people might be less inclined to try modifications to the game play is that a lot of players tend to prefer a finished product, rather than a work in progress that has issues. They may end up liking a finished product a lot, while being annoyed by the progress necessary to get there.
Having different kinds of servers, with specific purposes and expectations could be helpful. Like production servers that have stable and well tested game play, public development servers that has more experimental changes that would benefit from public testing, and that would still be good enough for public testing, and then private development servers could come in handy for testing things that might not yet be of sufficient quality for public testing.
Allowing for familiar transitional features can also be helpful. Like we are including the 1.3 Vanilla mode as mainly 1.1 vanilla game play but with miscellaneous fixes/enhancements to allow for better quality legacy play on 1.3. Interestingly, while we still have some more fine-tuning to do on Vanilla, a lot of old 1.1 vanilla players have indicated to me via feedback that they are liking the 1.3 Vanilla mode better than 1.1 vanilla, which is a good sign. But I believe that by the time the swirl mode and casual mode are gone, those would become more popular game modes than Vanilla.
I wanted to point out that for other (modern) games, the exact opposite attitude is becoming prevalent:
That is interesting. I did notice that on gpp at least for the last few years of activity, there was a preference for the actively developed mods, new game play updates were expected, and the attitude was annoyance when development didn’t keep up. It did seem that long time gpp players were generally more interested in playing other maps than atcs as well (even though an atcs based map was still relatively common on gpp, it was no where near played as much as on 1.1 vanilla servers). Also it seems that the players from 1.1 that enjoyed the 1.1 mods more than 1.1 vanillaish game play are also more open to game play updates, and to playing non-atcs maps more.
Perhaps the issue is based on the overall development activity of a game, and the regular players of said game. It is my understanding that most regular diehard 1.1 vanilla players that are still around have been playing that game play for at least the better part of a decade, in some cases longer, and 1.1 vanilla game play has not changed in 13 years.
What @romdos linked is pretty much what happened to tremulous over the years in my opinion, or at least an element of it. A lot of teams were created and tried to make big changes. And as soon as the big projects were done they had no plans for further improving or the plans were never realized. Some players got used to an active development process from such teams, got used to the new content coming out and as soon as the devs announced that they stopped working on such projects for different reasons or slowed the process, we saw a decline in the playerbase. (Not blaming them for the decline of tremulous, it was inevitable, but we cannot deny it helped on that process. Nothing wrong with it though, that is how such game work sadly.)
And as @dGr8LookinSparky pretty much said, people tend to like knowing when X update is due or when Y big update should happen. (Just take streamers as an example, the big thing that makes them so popular is their regularity. They stream everyday, at the same hours.)
A team that I absolutely love how they dealt with such problem made a video not long ago, I highly suggest watching it. A lot of their points where already spoken of here, but I just want to show an example of how well it did on a big scale.
From the creators of Path of Exile: (By far one of the most successful title currently, as they prove each time they release a new season with new content.)
Tremulous needs something that makes people want to come back after X time because there will be Y thing coming out and so on. But it doesn’t mean it has to be every 3 weeks or anything, simply a scheduled release/show time for those potential players to get ready for. For example, every few months could have some updates and every year could be a massive update/big update.