they were more consistent and eventually had more notoriety but they weren't ready for a P5.
When you play Idaho and SJSU, you can hold out your best plays and keep your stars healthy for the 1-2 higher profile games.
You can't do that in the more challenging MWC. Which is why BSU has struggled to make headlines at the next level while Utah and TCU are playing in multiple postseason games as P5s
Boise would need to show they could perform in a better conference like Utah and TCU had. Gimmicky plays and high performance offenses are fun but they don't impress the way Utah's win over Alabama or TCU's win over Wisconsin played out.
Balanced teams with NFL laden defenses. Utah and TCU had the challenge of losing sophomore and junior talents early to the NFL draft. That made it more difficult to sustain back-to-back top seasons. Something Boise didn't have to deal with.
But even then, expansion is about so much more and Boise only checked that one box. Utah and TCU much more complete universities.
BYU fans love the Boise argument because they don't want to credit Utah and TCU for accomplishing something they couldn't...and pretend that it had no meaning.
Notice, BYU has been passed up in expansion. For the first time in over a decade, they flirt with a top 10 finish, have a high profile draft pick and almost immediately get selected.