It recently dawned upon me that the reason for the extinction of the hybrid are the extensions of the talent trees. In vanilla wow we had 51 talent points to spend, our trees went up to a 31 point talent. This means that we could "Spec" to anything and still have 20 points to put in another tree. This was a ratio of 31:20 or about 1.5:1. In BC we have 61 talent points and trees that go to 41, this means our ratio is now 41:20 or 2:1. In Wrath this will be even worse with 51 talent points and a ratio of 2.5:1 for anyone who wanted a 51 point talent.
So in vanilla, specing as a hybrid was perfectly viable, however in BC in order to be effective we must become an arch type, in Wrath this will become even more pronounced with people putting an even greater percentage of their talent points into one tree. One fix for this would be to not extend the trees at all, this would increase the number of points we could have in our "off-tree" and allow people to play as hybrids once again.
I think that Vasc is basically correct in a lot of respects, though deeper talent trees are not the sole reason for the extinction of hybrids. Time costs, increasing number of stats, and gear specialization play large factors as well.
As well, 31/30 specs would be pretty powerful, so Blizzard had to make the new top talents even more powerful. I shudder to think how crazy the 51-point talents will have to be in order to prevent people from going 41/30.
I too would prefer that Blizzard keep talent trees at 41 points. Maybe change, clean up, and add new talents lower down. I think it would also allow for a greater variety of builds. Right now I find that there's basically 1 build for each tree, and there was a lot more build variety in WoW 1.0.
I doubt Blizzard will do this though. Too many people would be upset if there weren't any crazy 51-point talents.