I think it's likely two main problems:
1. Tanks jobs are intentionally made more difficult. Having to tab target to check enmity doesn't exactly make it easy on tanks. An Omen (WoW addon) type of interface that has the AoE mode would be the best.
2. Damage dealers more often than not don't really care if they're making it harder for tanks. For example, out of every 5 DD who attack the wrong target, only 1 will notice. The other 4 just keep on attacking the wrong target. Also DRGs and BRDs who attack the moment I run in, MNKs who are on-target faster than I am, etc.
So tanking already has a higher barrier to entry than other roles, add to that the unforgiving environment that tanks have to live through, is what's making people shy away from tanking.
I believe there are a lot of people wanting to try out tanking, but they don't have the opportunity to practice. They don't want to "waste other people's time" by practicing in DF or so. So in the end they stay away.
If the community is in general very forgiving of newbie tanks, willing to offer tips, then there are probably many more tanks now.