Note, the following is pure speculation on my part and I have no corroborating evidence. 

It's almost like Trump is deliberately allowing FEMA efforts on the island fail. He is a shady real estate developer after all so why not let everything fall apart so that most residents have no choice but to leave. Property values would drop to nothing. Then Trump can sweep in and buy up all the land he wants so he can develop beach front resorts for the obscenely wealthy and private golf clubs in paradise.

Nah, he wouldn't take advantage of a humanitarian disaster for personal gain would he?