Liberty had 10 scheduled games and said they were finished. So this means they were open to scheduling an 11th game, but with 3 P5 games for them already maybe they were not interested in a possible BYU game. Or maybe BYU thinks they will get Army that weekend.
Liberty has Coastal Carolina on Dec. 5th, so there are no mutually open dates for BYU and Liberty except for Dec. 19th. Maybe that would be too late in the year for them and BYU to play each other as the 12th games for both?