What does "6 feet tall" have to do with it? Hey, let's take a look at the teams that have been winning national championships:
Last year's Final Four MVP (Berry) as only 6 feet tall, the year before that Villanova won it with a tiny four guard lineup with the smallest guy being the Final Four MVP, the year before that the Final Four MVP (Tyus Jones) for Duke was yet again only 6 feet tall, the year before that the Final Four MVP (Shabazz Napier) was a guy only around 6 feet who totally carried UConn to the title; the year before that BOTH of UL's starting guards barely scraped 6 feet, etc., etc.
If you think height is a valid reason not to start Green, well then that's just dumb. The teams that win national championships virtually always have at least one guy around that height in their starting lineup (and often that seems to be the guy most important to the title run).
There are three guards on the team not named Brad Calipari. One of them is 6 foot, which limits your roster flexibility in ways that having a bigger guard wouldn't, particularly when he isn't our primary ball handler and we switch everything on defense. If you have three SGA's, you can play those differently than if you have three of Quade Green. This doesn't feel like a controversial statement.
Therefore, if you only have three guards on the roster, giving up on one of them and burying him completely leaves you zero flexibility in substitutions (unless you want Knox, who is already too slow laterally to guard SFs chasing around a SG), or in case of foul or injury trouble.
We've seen Quade be hit in the eye and miss games and have a back injury, which is the type of thing that often flares up again.
No one is arguing that Diallo is the better player. We're explaining why Cal keeps going with Diallo in order to keep him engaged mentally. If you bring him off the bench for 4 minutes a game, he's gone in case you need him. His confidence is already rocked. You're seeing Cal and the team try to build him back up. It's why they clapped and hugged him following the game the other night when he played well.
Cal knows his guys. He knows he can bring Washington or Gabriel in and they'll play fine. Other guys don't respond that way.