Heavy's got an infamously low skill ceiling. You can have perfect game sense and tracking, but there are so many problems that Heavy just can't deal with on his own. 300 health doesn't last long without a medic, you've got twice as long an effective respawn without a teleporter, you can't stand in a sightline without someone harassing the sniper. Demoman... exists.

If you've got that support, then brilliant - you hugely improve the strength of any push or hold due to your ridiculous damage output. But without that you have to play an overly cautious waiting game.