I believe it has to do with the condition to gain them. In the original set, you could gain a wonder as soon as you fulfilled the condition on it. In Renaissance, you put cubes on the wonders untill you have enough to claim it. Combining these diferent rulesets wouldn't work.
I'm not sure I understand why that wouldn't work-- the wonders in the original set seem to act like new wonders which only require a single cube.
Maybe that's bad because the new ones require multiple turns and the old ones don't take as long? But presumably their VP values are balanced for that.
So I am still curious why they couldn't be combined, given that the rest of the game is designed to be.