They don't want to play for a losing program. Those were good recruits, the team wasn't any good so they're leaving. That's true for every losing team in every conference.
Look at the winning teams in the Big12. Are they losing starters to the SEC? I haven't looked so I don't know, but I suspect and hope not. BYU hasn't lost any starters to the portal. Now go look at MWC, AAC, and FCS. They are losing starters to bigger programs because guys want an upgrade in money, exposure, coaching, competition, etc.