Well for starters, I feel that in general a lot of people just don't do commendations either because they feel people don't deserve them or they just can't be bothered to do them, doesn't make it right or even really wrong but it is what it is.
As for tanks getting comms a lot, that is just one side of the coin. Tanks both tend to get the comms when things go well or they tend to get the crap if anything goes wrong. They get the praise as well as the blame because of the player expectations heaped on the role's shoulders for better or worse.
As for me, I give it to whoever I feel did a good enough job to make me take notice, whether that is another tank, a healer or a dps. If healers keep my HP up and relatively stable while managing to help dps and stance dance, you'll probably get a comm from me.