That is correct. Few people main supports. Probably because everyone wants to just kill stuff and not bother with healing or aiding allies.

The thing is, it's very difficult to win a game without a support on your team. So most of the time people start arguing on the beginning of the match about who's gonna be the unlucky guy to be the support. And the one who decides to be the support almost always picks Mercy. Thats because the other supports are either nearly useless (Zenyatta, Symmetra), or too difficult to learn ( and still somewhat useless) (Lucio) (no offense to those who think otherwise). In about 60 matches I have probably seen a couple of Symmetra's and one Lucio. All the other matches were either with Mercy or no support at all.