As others have said we want to keep Azeroth inhabited post-60 and by leaving the Auction Houses and class trainers we can keep an entire game where everyone is interacting at some point instead of a large chunk of players just moving from new land to new land... Performance concerns are also obviously factored in as well... If you'd like to avoid being bothered you can use /dnd

The last thing Shattrath needs is more lag and more framerate issues for people who don't have great computers. "zomg get a good computer or quit" will usually end in quit, and that's not a thing that Blizzard wants.

This is the same song and dance we've heard in the past regarding AH in major cities. Remember when they were just in one city? Remember how Blizzard said there were "no plans" to add them to all cities? Then, as the months slowly rolled by, they softened their stance and eventually AHs were popping up all over the place... By the end of next winter, there'll be AH in Outland.