I'm thinking:
1.) to establish order and a consolidated chain-of-command admist chaos
2.) territoriality
3.) CYA - to stop competition from revealing FEMA's internal inefficiencies, deficiencies, and failings
4.) keep do-gooders from eating FEMA-paid contractors' lunches
FEMA is no different than any other Government Bureaucracy. The core mission and core competency of its administrators are to grow budget and to expand headcount; and to neutralize those who get in the way.
Bureaucratic mantra "Never let a good crisis go to waste."