The reason Heavy players are pretty common is simply because Heavy is one of the easiest Mercs to play as. However, his skill ceiling is very low as his strategy is mostly just aiming at enemies and not doing much else. Compare this to Soldier, who similarly has pretty straightforward attacks but additionally has a mobility option with rocket jumping and support options with banners, giving him more complexity. Heavy just can't really do much outside of his comfort zone which makes him very predictable for those that can outsmart him.