Kaioken questions.

So I’m watching DBZ(specifically Kai) all the way through for the first time and I have a few questions about the Kaioken.

1. Why does nobody else use it? The explanation I’ve heard the most is that none of the earthlings have a high enough durability to use it because saiyans have “more durability”. I have a hard time buying that one because I’m 99% sure Krillin and Tien are definitely stronger than Saiyan saga Goku by the Frieza or cell saga, and even if their not, Piccolo definitely is.

2. Why does Goku not use the Kaioken to get down snake way faster? If he was just going to take a senzu anyways doesn’t it make more sense to hit the Kaioken down snake way and go twice as fast?

Thank you everyone. Btw Krillin is awesome.