you can block a wheel kick or a knee to the head, or easier to get out of the way, and what's the percentage of Barboza landing one of those flush?
It's also greater risk and uses more energy when throwing wheel kicks or flying knee, not an easy technique and if you miss, the opponent can close in and do shit to you, which will prevent them from throwing in volume. and if they do land, usually a finishing blow so taking the risk is worth it
whereas sidekick to the knee is opposite of all of that. easy to throw and easy to land, not much technique or energy required, hard to defend, and other than giving permanent knee problems what do you get? not much, just keep the opponent away from you, also the percentage of those side knee kicks landing must be around 90%, I can't really think of anyone throwing that and missing the knee, have you?