Because with a pad you get an almost infinite adjustment for steering and acceleration without tapping a button like a mad man to achieve the same (but never quite as good) effect. You can hold the steering right or left at 10%, 30%, whatever%, same for acceleration making minor subtle adjustments with decreased chance of overcooking it.
I'l be VERY surprised if (for example) you can drift better with a keyboard than with a pad. The trigger for acceleration is an absolute must, on off on off on off with the throttle is no good at all!