Giving out extra compensation for tanking will only result in more people that can't tank running dungeons as tanks resulting in dps and healers being less inclined to be nice to new tanks, that in turn will make even less people main tanks and the spiral makes it's way downward. Is the answer to make it easier to tank? No, since right now it's not difficult to gain and keep threat. Is the answer to give people a bonus for playing as a tank? No, that will like I said only result in people unable to tank play tank simply to get that bonus... a bonus that I that main tank care little about.
Another fact that most seem to miss is that in every mmorpg there is always a shortage when it comes to tanks, the popularity of the roles goes like dps>healer>tank.