Why? Because it's a bad idea.
If you played Halo, you'd know exactly why. There is always only one most popular map in the random selection. Now that works if you have 20 maps to a degree, but it's still a bad idea from a development standpoint. Even if you are picking from well designed maps, in the end, developers painstakingly work on a map, only for it to be constantly voted out.
Devs and companies have no interest in doing work on something people don't play. Even if you just want to stand where they're looking from, if you think a map is bad, THEY don't think their maps are bad. Kings Canyon is my favorite example of what I think is a super trash map (all versions), but I promise you none of the devs thinks it's either bad or even if it was, unsavable.
Anyway, let's take that over to voting. Players just pick what's popular. If you are in the minority, and like a map that never gets picked, you're stuck always playing what other people outvoted you on. You never get to play your favorite.
Map rotation makes far more sense in that people share play on their most and least favorite maps. All the work Devs do and keep doing ends up paying off in their eyes. Map selection ends up reducing what people play on, learning how to play in new maps, tosses out work, etc. etc. I seriously doubt, with only 4 maps so far, and all the work that goes into each one, that you will ever see map selection. And, like I said, I think it's a bad idea.