I am no BI dev but here is it from a dev perspective:

Explosions and especially fire are very hard to realize in realtime games because it are in reality volumetric effects and the claculation power of modern GPU's is no were you would need it to be to calculate such effects.

More so explosions don't have anything to do with PhysX, PhysX is only for Rigid Body dynamics, which explosions and fire are not.