Why? He just does. Who else do you know who rides into battle when the other side outnumbers him 4 to one, and then rides in, reins in his teeth, firing off a winchester breech loader with one hand, and a Colt with the other?
Also, in the terms of the competition... The Vong have plasma, which burns biomass.
They also grow their own ships, and have propultion pods that can generate gravitational singularities. A Vong Coral Skipper absorbed an entire X-Wing. Just think of what a Worldship can do!
Also, they can attach one to the Tyranid ships, and set it to pull the nearest moon into the thing. That was how Sernpidal was destroyed... and Chewie along with it. (Chewie!!! You'll always be missed!)