Because (and has already been explained to you) vanilla’s gear system and entire post 60 progression system revolved nearly exclusively around raiding.
Every other source of epic level gear was purposely made to be more challenging, less available, and/or even less desirable.
Asking to change that, is asking Blizzard to change a fundemental design structure of vanilla… Hence my repeated suggestions that perhaps you would be more interested in the retail game, where your desire to be a non raider can still be met with epic level gear.
This is an objectively poor question that proves the exact opposite of the point you are trying to make, because there will be significantly more people wearing it, in part because it might become a part of the raider upgrade path (meaning more people in general are going to be trying to run it), but also because simply increasing the time duration for which it is available all but guarantees that more of it will appear in game over time, especially when you admit that the raid guilds could sell the runs. Do you really think guilds won’t be capable of doing the runs in slightly weaker gear? Or especially when they’ll already be chaining the runs themselves to get their own players the loot, since moving it up on the time line would mean it’s now part of their early upgrade path?
Which, again, is a completely irrelevant fact that does NOTHING to support your argument. Naxxramas being harder than Molten Core isn’t a justification for Naxxramas to be available at launch, and it your argument fails via the same exact gap in logic.
Finally, and I hate to drop a reductionist statement on you, because there are things that are quite arguable for potential changes… but