Well, If all of your death school friends reached legendary(level 60) then I suppose they are playing their other wizards. They may occasionally log on to their death wizard to craft something, but untill the level cap gets raised again, or untill new content is added, I doubt your friends will be playing their death wizards.
If you meant why are their fewer death school wizards period. Well death is such an awsome school! I know I picked death for my first wizard. Again, when people hit the level cap they tend to switch to a different wizard. So the players are still there, they are just using different schools.
Um, how do you know how many death wizards are being played anyway? I mean, people go reagent farming, boss farming, they go to help their little brother so he will want to play too, and sometimes they pull an all night play session and leap ahead of you. I know my death wizard wears silver and white, he is often mistaken for an ice wizard. We also have several channels and servers It would take forever to check all of the locations on each, "relm," and count death wizards. You cannot be everywhere at once, so how do you know there are fewer death wizards on than there used to be?
Well, maybe because you were on "realms" that are Perfect and Normal. Also, mostly 1/2 of bosses are Death. Which is hard for Death Wizards. But now, Death Wizards found a way to fight Death Bosses. Now thats why you see more Death Wizards.