The "K" in "K-rad" means kilo, or 1000 (or 1024 in the computer world) times as rad. Hence, K-rad.

K-rad came about in the 80s when the term "rad" (short for radical) was popular. Computer geeks decided to make the term "rad" geekier by making it "k-rad", so they'd have a term of their own.

Even though m-rad and g-rad (mega and giga-rad, respectively) would be even cooler than k-rad, theoretically, they have not caught on, probably because they don't sound as cool.