Bowser for one reason and one reason only: Fire.
In the games, setting a Tank on fire results in an inevitable death for the Tank as he slowly burns to death. Nothing the Tank does can ever extinguish it.
As soon as the fight starts, Bowser is going to open up with a fireball, and if the Tank gets hit by it, which he certainly will at some point in the fight, he is guaranteed[i/] to die. If they fought, say, 1000 imaginary battles, the worst that Bowser could do in any of them is tie, since the Tank will die shortly after the fight is over.
Bowser is going to win at least some of the time, no matter how outmatched he is, due to the laws of statistics. If he wins just once out of, say, one million matches, his win percentage is guaranteed to be higher than the Tank's, rendering him the statistical winner.
It's just science.
(I need a job.)