So, as normal, a multiple-cause accident.
If Scott Crossfield had been requesting weather updates he would have had a more complete picture of the weather ahead, and could have "worked around" the worst areas... instead of flying straight into it.
If the ATC had done his job properly, he would have warned of the supercell activity and directed a flight path around it.
EITHER action/person could have avoided the crash... it took inaction by both to kill a flying legend.
As the Naval Aviation Safety Magazine (USN) says in every issue: "Break the Accident Chain"!