The main quest has nothing to do with the other factions becoming accessible; it's when you reach level 50.
Wait... so you can only go into other territories when you hit 50?
Cyrodil is the only pvp area in the game.
Well that's just retarded. You can't have a PVP game then say "oh yeah, you can't actually attack a faction in their home territory".
Not everyone who plays MMORPGs wants to be doing PvP literally all the time. There would be no way to concentrate on the PvE material in your home territory if all the while there were mobs of "invaders" running around ganking everyone.
There are other ways to deal with it such as PVP flags. WoW does this perfectly. Don't wanna PVP? Don't put on the damn flag or attack another player with his/her flag on.
And in my experience, if the world is big enough, the mobs of players running around ganking is pretty light. Especially if the endgame PVP in Cyrodiil is engaging and you get no points for killing other players outside of Cyrodiil.
I wouldn't call it a "PVP game" since there's only one PVP area.
Which kinda defeats the whole "three factions at war" thing. If all they can do is fight over one area then it's not exactly a war. More like a game of "king of the hill".