>>472142303 main reasons.
Leech Seed gives it momentum if you switch, Iron Barbs gives it momentum if you leave a physical, contact attacker in on it that doesn't have a Fire attack or something.
It carries all three of hazards, status, and damaging moves; it isn't passive
Its x4 weakness is super telegraphed, and it's bulky enough that random or unexpected pokemon carrying fire moves still won't kill it, it still has to be a pretty respectable physical attacker.
The real kicker is the Leech Seed T-wave combo, you can't set up on it
