Apologies for the cross post but this was my explanation from another thread:

The reason most people don't want to play a Tank or Healer is down to the community, same problem I had with WOW, EQ and a few others. The Tank & Healer are just as essential as the DPs for getting through a dungeon, however the moment something goes wrong the tendency is to blame the Tank or the Healer for not doing their job properly. That discourages people from playing that class and leads to a shortage of people to fulfill the role, very quickly those people who do stick it out are scooped up by guilds and stop doing PuG runs. Which means that the general ability to get into dungeons for low level PCs drops. The only way to fix it is for the community as a whole to change and we all know that's about as likely as a completely bug free release.