I've always thought an unwritten gentleman's agreement was reached. The Big Sky took in Davis and Poly football, and the Big West left Sac State alone. There is nothing official substantiating this, but it would explain why the status quo lasted for 15 years. Look at what former Big Sky commissioner Doug Fullerton said.
The Big Sky Conference is proud to announce that the University of California, Davis and Cal Poly have accepted invitations to join the league as affiliate members in the sport of football.
bigskyconf.com
“We worked in cooperation with Big West Conference Commissioner Dennis Farrell,’’ Fullerton added. “The Big West fully supports this effort, as it gives its two football-playing members a home.’’
Kinda makes the unofficial understanding between two conferences plausible. The "gentleman's agreement" only ended when the MWC added Davis.