Of course... if we never publish a 21st-40th level book... then level 20 does sort of become the limit. And unless there's enough demand for an epic book, we won't print it. Despite the vocal support epic level gets (and I am part of that group who likes epic level), the overall picture of how popular epic level play is has traditionally been pretty dismal.
It could be just because Epic Level rules... suck.
But I think the reason is that people just don't usually run campaigns lasting long enough. Most start at a lower levels (if not 1st, then 3rd or 5th) and can get only so far because the group scatters, the DM loses interest in his campaign, or the players just wants to try something different.
Some people will still play till epic levels - some might even start a campaign at epic levels, but it's just rarer.
A reason for that is because: What to do up there? Really, what kind of stories can you tell if everyone involved has demigood or god-like powers?
If you are still just looting dungeons, what's the point? But how good are you at epic level politics or world changing?
You are no longer someone that just "reacts" to stuff - you
have to become active. As Mr.Morden asks: "What do you want"?
I think that is a kind of change of play that is harder to fathom then just moving from killing goblins to killing dragons to killing demons to killing gods. What do you do when you're a king, what do you do when you're a demigod or a god?
It's also something D&D worlds seem to be generally bad at. Most gods seem passive. Some evil gods might plot stuff, but usually it's just their followers doing thing (like getting their god out of imprisoment). There is no "role model" for what epic creatures do. That top tier of "gods" is often presented as the stable and static campaign background. But players are not campaign background, they are foreground.
Jürgen Hubert mentioned this on this board and his blog - his experiences with Exalted suggest that its the players that drive everything. They must have a goal. They must change the world, and as epic individuals, they must change it fundamentally. It's not just founding a kingdom (which would already be pretty cool for a "normal" campaign), it might be about changing the fundamental rules of the world, shattering cosmology or at least the hierarchy of gods.
It is a change both for players and DMs. The DM is used to have a more or less stable background. He is in control of the world, but he has to give up a lot more control then usual. The players on the other hand, they have to take matters in their own hand. Not everyone is used to that, especially not to that degree.
Of course, these are all generalizations. There are players and DMs accustomed to such scenarios.
But if we look at the game materials - especially modules - that is out there, there is little that supports it. Primarily of course because if it's the players that drive the plot, a module is less useful. You'd need something like a "ideas of how players can shake up a campaign world and how you help them do it".
But even with such materials it remains to be seen if there are enough people interested in it. (I know that earlier D&D games had rules for stuff like fiefdoms and similar aspects that went well beyond 3E style Leadership feat. But I don't know how much of it was actually used.)
If not, it might be better to find a different type of "advancement" at some point. Instead of getting tougher, stronger and faster, characters might just get more versatile. Fighters don't become better Fighters at some point, they branch out and become Wizards. Wizards don't get more powerful spells, but they find religion.
