Because you don't know anything about Swamp Thing?
He can control plantlife and prevent Man Thing from reforming himself as he tears him apart. Plus he's way the hell harder to put down, being one of if not the only being to survive the antimatter wave during CoIE.
The only thing Man-Thing has going for him is that whole "whoever feels fear" thing but why would Swamp Thing be afraid of him? And even if he was, I really doubt he could burn him enough to put him down for the count.
Well Man Thing is hard to put down because touching him hurts you if you feel fear, he's a pretty tough dude and because he can reabsorb plantlife into his body.
Swamp Thing won't be scared of him at all so there's one problem down. Swamp Thing is a pretty dang strong dude so there's problem two taken care of. Finally, Swamp Thing has a powerful influence over plant life. Though I don't think he can just make Man-Thing melt or anything, I believe he could prevent him from using any vegetation to regrow himself. So it's possible for Swamp Thing to beat Man Thing.
On the other hand, Swamp Thing ridiculously hard to destroy. As in he can reform from plantlife elsewhere after being erased by the antimatter wave. Unless they fought somewhere with absolutely no vegetation, it would be next to impossible for Man Thing to beat him and even then it would only be because he destroyed his body and forced him to reform elsewhere, winning by bfr.
But Swamp Thing is enough stronger than Man Thing that even under those conditions, MT wouldn't win in a straight up fight.