Because white usually is better versed on the theory than black. The biggest
problem with the KID is all the theory you have to know. White just needs
his own pet line. Curious, in a tournament in January I played the Bayonet
three times losing twice as white and winning once as black.

StanB

Most likely because everyone is booked up on it as black. However if
you just switch to another line you'll probably end up with good
results. I think the KID is just not good to have as a number one
defense because you have to devote too much time to it. Fischer even
realized that then Kasparov eventually.

EZoto