it's mostly because they have more than enough content below 50, people seems to fail to see that most of them time in a mmorpg will be spend at max level... some people like to level up more, but they are a minority.

why add content that will have little use, when add content level 50 will add more stuff to do.
other point all this level 50 dungeon will be handy when the expansion and the level rise will come out ^^