Here are a few reasons I can think of right now:
-Hype
-Lordaeron had for a time better script then Icecrown (+ Icecrown was two servers that merged -> hype, because of the merge + Icecrown got the same core as Lordaeron).
-A lot of people don't like to play on Lordaeron, because there are no RDF.
-Some people don't like slow leveling, so after they are tierd either of just leveling their first, or their alt, they quit.
-Some people don't like that the raids are buffed (or how much they are buffed - note: They have reduced the buffs).
-I guess some people are waiting for ICC? - only time will show.
(Not every reason fit everyone, and some people got most likely some other reason too).
Will the population increase when ICC releases? Who knows, no one can give a secure answer on that. I am pretty sure that there will be some hype, and the population will increase for a time, but will people stay - thats an other question. From earlier experience on "patch" releases, there has been a population increase for 1-3 months, then it drops again. But who knows now, when ICC is the most wanted one?
I guess there is no answer, only time will show.