It's everything. Natives used to practice controlled burns to sustain habitability. Observe parallels to three sisters agriculture. Then as their populations were eradicated these methods were lost.
Indeed. Though modern fire suppression is a huge factor. AFAIK fire suppression became industrialized following world war II (bomber type planes able to deploy large amounts of water, better radios and respirators). Even today, the mandate for fire suppression (in at least WA state) is all fires are to be put out within 24 hours. There are a few "let it burn" examples I'm aware of in CA
