The BYU players who get drafted have the measurables. And it is as simple as that.
As to Gonzaga, their best players are all transfers from over P5 schools. WIth a few exceptions. And if they don't get drafted or play, it is about measurables again. Pro teams don't care where a player comes from. At all. They just want players that get help them win. Just go look ta rosters. Plenty of players from schools smaller than BYU on rosters because they have the measurables.