RINOs won't let Rick Scott be majority leader, and Thune and Cornyn are Mitch's little lackeys. They won't let Mike Lee do it because he's too mean and conservative despite being anti-Trump until very recently (in that Meatball/Chip Soy vein of acting tougher than what can actually be accomplished).
I think Rubio would be a fair compromise for majority leader, it keeps him loyal and he's not as divisive as Zodiac or as fake as Tom Cotton. Rubio is smart enough to know he does Trump's stuff, he will be more liked and have a better future.