In regards to the seemingly sudden expansion of not only the reported range, but also of public interest/awareness, one might consider the state of information delivery and its relative evolution and progressive effectiveness over the period of time in question (1950's to present) as this may well play a significant role in understanding the dynamics of the phenomenon. Television was just warming up, few national networks, papers were largley local in relation to todays(well..newspapers...yeah)and the internet was but a glimmer in its daddy's eye...
There was little means for people in a given region to hear reports of these creatures in distant locales, if the locals were even relating such experiences to media outlets, more likely they kept mum due to potential ridicule, thereby preventing accumulation of sighting data and development of more comprehensive grasp of possible habitat viability and range.
As we all know, the first nationally reported account was the bluff creek/jerry crew event, and that was in part due to a set of circumstances that lead to the article being pick up by the national press. Maybe by making up a new name, along with no image of the creature, widespread recognition, that "hey! Thats kinda like the quarry trolls we used to see when we were kids" or "wow! That looks just like the footprints out by the petting zoo..." was hindered, as each region had its own names, local lore and superstitions for these creatures that few chose to speak about openly in the first place. But that hardly means they werent there in the various regions and habitats before becoming known in the mainstream, the info just wasnt getting out, gathered, or put together as representing a single actual type of creature, but rather folklore to keep the kids in at night, or to scare the smug city dwellers that arrive on occasion, thus discredited and dismissed.
But over the years technology advanced, media coverage became more comprehensive, accessible,widespread, pervasive, influencial, cohesive/consistent in terms of content presentation and selection, and controlled by progressively fewer individuals/groups. This sallowed for the discussion of sasquatch to develop from thinking of them as only existing in the PNW, to realizing that allthese regional monsters and boogers all across the continent may actually be the same critter. Then the first brave few began telling of their experiences, and despite the ridicule, others then spoke out. And the media ran with it, and over the decades, transformed the image in the eyes of the public, all the while never actually confirming its existence.
There is a parallel in what might be seen as a rise in domestic abuse or child molestation within our society. However, i'd bet its more a case of media coverage/discussion coupled with those who broke the silence and dared to speak openly about these horrible aspects of our lives. You never saw june cleaver talking to the boys about rape from the woman's perspective, but over time these issues entered the national dialog, despite ingrained beliefs that one simply didnt talk of such things. Now days these topics are addressed 24/7 in our media in virtually every context and format. Now is sally jessie raphael responsible for rape being so prevalent? .org course not! These things have most likely been with us since before we thought the first floor (ie the savannah...out of the trees..)might be an ok place to raise a family, and only relatively recently have they been addressed openly on both personal and national levels.
This may be a similar situation as the sasquatch explosion that you seem to think disproves their existence, when it is indeed a gradual acknowledgement of at least the possibility that there may still be substantially significant elements of our planet that we have yet to grasp or come to know, one of which is our closest extant relative,(who never calls.... or writes...)sasquatch.