Quail populations where I grew up were fantastic in the early 70's but by the time I was old enough to take myself and have my own dog (mid 80's) it was hard to find enough birds to make it worthwhile.
The fields hadn't been made any bigger in that period of time, still plenty of cover.
Some blamed the newly acquired turkeys for "eating the quail eggs", some blamed hard Winters or overly hot Summers, and others blamed foxes and coyotes. But I don't buy that there were any more coyotes then than there was before. Probably LESS if anything because everyone that saw a coyote/fox would shoot it.
Nobody ever blamed hunting pressure for the lack of birds even though at least 3 families per block in the town I grew up in owned a bird dog or 3, and all those ol'boys were wicked good with a shotgun.