Why everybody wants me to switch Rocket in Comp?

Okay so this is the third time in Competitive (Gold I) when my team forces me to switch Rocket to any other heal. One time somebody said that he's "one trick pony" and the other time they said he's the worst support. Why is that? Rocket is the best healer and he can revive teammates, I don't get it at all. Could somebody explain? My morale is decreasing hard because of such situations...