It's because the general 'online community' is completely retarded in every way. If you've ever played an MMO for a decent amount of time you'd have a crystal clear window into the problem.
The very people who complain about a lack of Valve maps have no problem playing crappy killboxes and rats knockoffs, choking good and near-valve quality custom maps out of the 'market', all the while still complaining that there aren't enough good maps. It doesn't really matter cause in the end all the servers that run only or mostly custom maps will be empty and the 24/7 2fort+whatever fun map tf2 gets will be packed, and Valve will realise all the hard work and design they put into Hydro was a waste when all they had to do is make a big empty box with random floating platforms to entertain the masses.
Just look at the Halo trilogy's level design, both sp and mp, and then look at its huge popularity and how it has become a 'cultural phenomenon' according to the New York Times or whatever, and you will see the dire straits level design is in.
