There’s less leverage
There’s less leverage propping at the knee than the ankle properly of course, but when you beat the fuck out of someone with Tai Otoshi on the street you don’t really need max leverage. If you’re a Judo BB and the guy is untrained and taken by surprise with a Judo finishing attack, you’re pretty much gonna win the fight.
Propping at the knee greatly increases like the likelyhood of an injury at the cost of some leverage.
Stop acting like if I showed up to your BJJ gym and started Tai Otoshing some blue belts hard that you wouldn’t ask me to stop, you would.