Well, for one there's little appeal in even trying epic play, no matter how fun it could be.
-Once you've reached level 20, you most likely "have done it all" (bar killing powers, which is the crux of the current big argument). You've killed the dragons, visited hell, destroyed lichs, saved the country, saved the world (often multiple times) and sometimes even the multiverse. You've been entangled in politics and detangled yourself. 20th level, for me, sounds like the perfect point for "ok, next campaign"
-D&D already has plenty of "real ultimate power" already from 10 level forward. There's very little you can't do once you've reached 9th level spell. Most things you can't do are still well inside your powerlevel, but just not covered by the rules.
-Number crunching ever increases. I'm not to cry- omg, teh math killez me!!1one, but from 20th level on it simply gets a lot, and it simply doesn't appeal to everyone.
-If your play is more episodic, suspense of disbeliev could be hurt. Why are all these multiple planar spanning ultra threads comming up just now.
-The system, no matter how great it may play, doesn't exactly look appealing or well done.
-Related to the "done it all" a lot of epic play looks like "what you already did, but bigger!!111"
-There isn't really an appealing history of epic NPC's. The biggest epic NPC setting is FR, where they are seen more as annoyance or major weakness of the setting. To those that don't see it that way, they are more or less holy or untouchable NPC's. Many people with at least a little D&D experience have played or DMed FR. Many of these had frustrating deus-ex-machina, plot-stiffling or suspense of disbelieve hurting expiriences with those super DMs. I don't mean to bash FR, I still think it had it's success for a reason, but it hurt the image of epic levels.
-For some reason, it simply doesn't connect. It seems so...unexciting for something called epic.
There are other reasons.
All classes are all presented in the twenty level model. Epic stuff is always locked away in it's own area and often looks like an afterthought.
Really, the epic rules are only in the 3.5 DMG because there's no 3.5 epic book.
The game itself is in almost every way a twenty level game, casually looked at. It doesn't only give an almost complete expirience in twenty levels, but also creates the expectation of a complete experience. If epic was to be base assumption, why not remove the level cap altogether. (Bad idea, imho)
A lot of all this is prejudice and lack of promotion by wizards, but there are simply good reasons as well, I think.
I guess my main argument is that it's simply not needed. Like I've said, D&D 1-20 provides an almost complete expirience thanks to the four power levels (1-4 gritty survivalism, 5-9 fantastic fantasy, 10-14 superhero fantasy, 15-20 sorry, but that's already uber epic in my book). It gives all most people want and in many cases a bit more (just look at "sweet-spot" polls and up to what level many people play). Why chuck out the cherry of the cake, the very top of the heap and say "No, you've got to go epic with that"/eat a complete other pie.
At 20th level I've killed the orc, gimme ma pie.
) even run games to 20th level let alone epic level.
