It's because the amount of realms there are and Blizzard refusing to lower the amount. This amount of realms was once there for almost 15 million players, theres half of that now and still the same amount of realms. That means on average each realm has had it's population halved. Then add the fact that most of the players flocked to a handful of big realms and you get why there are so many absolutely dead realms around.
On US servers there are 12 realms with more than 150k players on (the realms where everyone flocked to) and 215 realms with less than 100k players (the realms where everyone flocked from). The lowest populated relam has 16k players, the highest populated one has 232k, enfin you get the problem.
http://wow.realmpop.com/us.html
The EU realms show exactly the same pattern, highest pop 220k, lowest pop 12k. 6 realms over 150k, 242 realms under 100k.....
http://wow.realmpop.com/eu.html