keeveek: Because Obsidian managed to fill New Vegas with interesting characters and plotlines on the same engine.
Well, I wouldn't say the reason is money really, I'd say the reason is development time. And there's actually a huge difference in how big the worlds of Morrowind, Oblivion and Skyrim are, yet they retain the same 'qualities'.
As for New Vegas, again, you're right, characters are much more interesting and it's got great plotline - on the other hand, world is fairly boring to explore and as in all Fallout games, there's only ever one 'occupation' that you can actually be for the game to be any fun. See? You just can't do it all, you have to sacrifice one thing to get another. In a way, Oblivion have tried to sacrifice world 'size' - sheer size of that game is massive, but most of it is either randomly generated or slapped together from various building blocks. And indeed, questlines did improve quite significantly when compared to Morrowind. (Oh, and cities. Oblivion just has the most detailed cities I have ever seen in an RPG.) But it'll never be on the level of New Vegas, as something else would need to be sacrificed yet again.
Morrowind and Skyrim are completely hand-built, but you can see the quests suffering, especially the guild plotlines.
keeveek: By the way, when does the Oblivion get too easy? I'm level 9 and I still need to shoot just as many arrows on certain enemies as on level 1...
Level 15-ish, when your fighting abilities get above what the game expects them to be.