^^But there are so many franchises where audiences are accepting of multiple contradictory "realities." People accept that comic, film, and animated-series adaptations of Batman, Spider-Man, etc. are in different continuities. They may whine that the latest Batman series isn't a continuation of the Bruce Timm version, but they don't whine that the Timmverse contradicts the comics continuity. The existence of multiple incompatible versions of the same universe is accepted in many cases, so why is ST so different?
That's a question with many answers, and no single one is going to answer it completely.
Most fundamentally, is ST really that different in that regard? How many people really think "Kirk commanding the Excelsior" comics from between STIII and STIV "should" be in the same continuity as those movies, vs how many can accept them as their own little bubble of continuity? Even with novels, plenty of people consider the "Shatnerverse" to be BS's personal sandbox and don't sweat the differences between it and other novels. And since it was known up front that the "Crucible" books wouldn't be "tied down" by other novels, was there that much grousing about them?
Even with those caveats though, I'll agree that "excessive canonism" is at least more visible within Trekdom, whether or not it's actually more prevalent. I think part of the differences between ST and franchises like Batman or Spiderman is that those continuities follow shifts in the medium (comic to movie or animation) or major shifts in style (i.e., the various Timmverse series vs. "Men in Bats"(1) or "Teen Titans Go"). And while there may be allusions to events of another continuity, they don't directly reference events from another medium. Just about every licensed Trek work references the events of the series or movies, creating some form of continuity.
Another factor in the greater visibility of "canon nazis" within Trekdom is a matter of timing and opportunity. In the Beginning, Trek was the biggest game in town for this sort of thing. Space:1999 didn't catch on as much, the original Galactica didn't last long enough, etc. There weren't any readily-visible counterexamples, so it was up to Trek to blunder around and make the mistakes that later franchises tried to avoid (e.g., Lucasfilm saying "it's all 'official'" even if they steamroller anything that gets in their way later (say hello, Grampa Lumpy) vs. the oft-noted different path taken with Trek regarding licensed works). Arguably, making that sharp distinction between "canon" and "non-canon" only made the problem worse -- those with the kind of personality that need that "offical stamp of approval" were handed a cudgel to use in their little turf wars.
These days, after two or three decades of Crises of Infinite Earths, remakes, reboots, "re-imaginings", Ultimate-ing, etc., the very concept of multiple continuities is simply more widespread and accepted. It's not just a "cheat and a cop-out" any more.
(1)So named for the stylistic similarities to the "Men in Black" cartoon series.
