Even with all the posts saying that the cities of Azeroth have started to become ghost towns since The Burning Crusade's release, some players seem to be homesick for the old world. Homesick enough to want to take their flying mounts with them to the lands of Kalimdor and the Eastern Kingdoms.

This has been a long debate and Blizzard has given a long explanation for this.

One: Flying Mounts were made specifically for Outland.
Two: Keeping them in Outland gives a special awe for the players, hence keeping Outland unique.
Three: To import the flying mounts to Azeroth would mean considerable time. Would you rather Blizzard devs work on porting the flying mounts to Azeroth than working on new content?

Blizzard concluded with:

Time and resources are two things we are very careful about and as you can see from reading the forums, a lot of people believe we should be doing more with it no matter how much we actually do. We do the best we can to put out fixes and content as quickly as possible but we have a constraint we've put on ourselves as well that if we are going to put something out there, that we should do it well and with Blizzard quality in mind. This is something that will always be at the top of our list to strive for.