In terms of the impact of the arrival of europeans, you can't diminish the probable damage done by the foreign antigens, which may very well have killed off the the vast majority of sasquatch, at least those within close enough proximity to the natives as they became afflicted. The Europeans practice of laying blankets infected with yellow fever and the like across trails could have transmitted these diseases to the Bigfoot just as they did to the native americans. It may have taken longer to pass between them due to their more widespread distribution. There's estimates of 90% mortality rates amongst natives (even higher in south america) if this applied to the sasquatch, perhaps their rebound was slow, then as whites killed off the majority of upper level predators, and deer populations began expanding, eventually to the point of overabundance, then the sasquatch had plenty of prey, little predator competition, and more empty land to move into, reclaiming old turf, thus expanding their numbers once again.