Is there anything special about rotor bolts?
The Torx heads are generally lower in profile than equivalent bolts with a hex socket. This solves clearance problems in some fork-wheel-disk setups. 

Torx heads can be made shorter because the star pattern of the hole provides more contact points with a screwing tool, thus allowing reaching the same torque values without risk of stripping the bolt head. An equivalent sexagon head would have to be higher to reach the same torque value.