I don’t think this had much to do with PAC prejudice against BYU. (Not in those days)
Let’s backtrack. In the early 1900’s BYU’s administration banned football! It was reinstated in the 1920’s. Most football seasons pre-Lavell were mediocre to abysmal. I know there were a few good years.
I think BYU in the 70’s was seen as a flash in the pan by outsiders. ASU dominated the WAC for many years and Arizona grabbed its coattails to get into the PAC.
It was extremely exciting to watch the early Lavell years. The GOOD football and eventually the stadium having to expand. I just don’t think BYU was taken very seriously nationwide. Later, wins like A&M, SMU, and Miami started to change opinions.