Interesting.
I think that after 30+ years of "someone's got to play the cleric" we're seeing some backlash. Regardless of how well they're built in 4e, "healers" and people wrangled into playing healers have a new option (and notice that's now the most popular choice.)
Or it could simply be that in a more secular (or at least, monotheistic) world, it's a lot harder to relate to clerics... we see action movies with warriors, soldiers, generals, tomb raiders, and even assassins; caper movies with thieves and con artists; fantasy and superhero movies with magic and powers... but generally speaking, culturally, we don't see a whole lot of "messenger and champion of 1/10th of a pantheon of gods." Notice also that Paladin is the second least favorite choice after cleric.
Probably some combination of both.
Other factors might include alignments being less important mechanically, and a shakeup of the planes to some unknown extent.