The Western Open was considered a major championship by players and reporters through at least the late 1940s.
Ben Hogan's win at Sunset CC in St. Louis in 1946 was reported as his first major championship. It came a few weeks before the PGA. And when he won at Brookfield CC near Buffalo in 1948, it was the third of three majors he captured that year, following the PGA in May and the U.S. Open in June. He posed with the Wanamaker, U.S. Open and Wadley trophies to commemorate the feat.
The rise of The Masters, and the penchant of the WGA to take its championship to such outposts as Davenport, Salt Lake City and Buffalo, more than a bit out of the way in that era, the reluctance to raise the purse above $15,000 until 1956, plus the failure to gain a national TV contract until 1963, all contributed to the Western slipping a peg.
Oh, and Mark McCormack, a native Chicagoan, decided it wasn't a major when he sent Arnie to the British Open in 1960.
There's more, but you get the idea.