Because more colleges would like into the P5 conferences then will get into a P5 conference. BYU now has one more thing it can place on its resume that the following teams cannot:

UCF
Cincinnati
Houston
Boise State

UCONN
East Carolina
Memphis
USF
SMU
Temple
Tulane
Tulsa
Air Force
Fresno
Colorado State
Nevada
UNLV
New Mexico
San Diego State
San Jose State
Utah State
Wyoming
UTEP
LA Tech
Appalachian State

and many more.

It is always good to have something on your resume others do not. Sure this is not the deciding factor, but it is one more factor in BYU's favor.