Numerous reasons.
First, learning new rules takes a long time. I like to have a complete understanding of the game rules, but that takes time. This becomes a problem as the edition gets more and more bloated, and it's even worse if I'm GMing (because I'm expected to know some bizarre new feat or some nonsensical new prestige class that almost nobody asked for). Then the edition is abandoned by WotC or Paizo (or whoever) and I'm asked to do it again.
The same issue impacts adoption of new RPGs entirely. If I already know a game system that can handle sci-fi, do I really want to learn another game system that can also handle sci-fi? Not if the first game system is "good enough".
I've found this issue different editions of other RPGs. I started with Fate: Spirit of the Century. Then Diaspora, which made for great sci-fi gaming (I only ever want to play Fate or Mutants & Masterminds for sci-fi gaming), but learned later the Storyteller (GM) had actually dropped some rules that I'm glad we never saw. Then we tried Dresden Files, and things started getting worse. Then Strands of Fate (superhero themed), which fell apart after only two or three sessions. Each version of Fate seemed to be making things worse compared to the previous version.
Second, when you like an edition, the newer edition may not have "what you want". I'll use my own gaming experience as an example. I started with 2e. I didn't actually like it much, with some things, such as the rolled ability score system, driving me up the wall. The rules were not well-written (using 3e-style gamist language) and there was little unified math, so people were forever looking up the saving throw charts, the turn undead tables, and the like. The moment 3e came out I jumped, as fast as possible. During the "interregnum", when 3e had come out but my gaming group had not switched yet, I tried (and failed) to convince the GM to allow Magic Fang as a 2e spell. I hadn't even played 3e yet but I was already seeing things the new edition was doing better, and wanting that.
So 3e came out and we adopted it. Pretty quickly, we were using point buy, and I refuse to have rolled stats. In addition to the lack of rolling, the ability score scaling made sense. Do you know what was the difference between Con 8 and Con 14 in 2e, without consulting the chart? I have no idea. In 3e? Con 8 gives you -1 to hit points per level and Fort saves, Con 14 gives you +2 to those instead. I was also delighted that "exceptional strength" vanished, that classes gained levels at the same rate, and so forth. But 3e still had problems, mainly in terms of game balance between characters of different classes.
Then 4e came out. I bought the three core books and started running. Fighters no longer seemed boring. Mages were interesting but not overpowered. Clerics had a role beyond healing and condition relief (without poaching other character's turf such as buffing to be stronger than a fighter, blasting like a wizard, using Hold Person like a wizard, and so on). It's my favorite edition. Of course, it still has its own flaws.
Then 5e came out. At this point I was getting older and had less time. We played the first playtest document and I noted the lack of 4e's mathematically tight game balance. I could not predict ending numbers. This brought back bad memories of trying to run 3e. I haven't made the switch, and unless Pathfinder and 4e gaming completely disappears I won't make the switch. The longer I avoid a switch, the bigger and more bloated 5e gets, which means the barrier to game mastery simply gets higher and higher for me. By not wanting to play 5e as soon as possible, I've essentially discarded it as an edition.
