Firefighters were called away to other fires around the island. That's from an eyewitness who was in close contact with Lahaina firefighters. Yes, grassfires are tricky.
I've had an odd career in the Marines, in that I have fought fires (using my troops) all over: Massive brush fires at Camp Pendelton, range fires in Oklahoma and Hawaii (Pokakuloa), big tree fires in Washington State - that last one with a 500-man battalion.
It would seem to me that they should have prioritized any fires near populated areas.