Because it's intimdating, say that you are a new GM, and you show an interest in the Forgotten Realms setting, you buy the book, sit down with it, and realize that it's too much to all take in.
In Podcast No. 22 three designers talk about the 4E Realms and a need to make it more accessible to new players. 3E Realms did have a considerable amount of back story, which could be a burden. However, simply advancing the time line a century would have been an effective way to deal with the issue. Draining the Inner Sea, destroying Mulhorandi, Unther and Halruaa, killing gods and similar changes feels random and unnecessary.
One of them comments on changing Thay to make it more like the Plateau of Ling, to get rid of cities, roads and so forth. My interpretation of Thay was always Communist Russia or Nazi Germany rather than Plateau of Ling. That way it could have cities, roads and so forth and still be an unpleasant place, so people not from there did not like it.
These days, Grayhawk appeals to me.