The game lacks tanks because..
1) They're very boring to play, especially Paladins. Yes, I know, DDs use the same rotation over and over too, but there are more skills to use and generally you need to be much more active.
2) Their faults are very visible and they get the most flak because of it. It's dangerous for a healer or DD to die too, but tank's death has the most impact upon the party.
3) People expect you to be informed in all fights that matter and tanks are the 'leading man' in those. DD and healers can get by with playing reactively whereas tanks has to play proactively. This puts extra pressure on tanks.
.. and these discourages people from playing them.