Willravel

Several reasons:
1) They're tough as nails now. While rotary automobile engines have in the past been susceptible to gasket issues, they're now less likely to die than piston engines.
2) Weight. Rotary engines are a lot less heavy than comparable piston engines, which is good because a rotary taking the explosion needed here would need to be a lot tougher, which leads me to...
3) O2/H2 explosion. This would mean a lot more power and a lot more heat than a piston engine can take. This means that the rotor would need to be iron. Better an iron rotor than iron piston heads.