I'll tell you why. When it rains, it slows the game down to a crawl completly.
First play on a completly dry field, then play in the rain. you'll undersatnd In the rain if you're the QB you cant grip the ball same for ball carriers you become more careful.
As far as the old wives tale that the offensive guy knows where he is going and the defender does not, yeah he knows where hes going hes going down the ground if he does not slow his ass down to try and make a sharp cut.
As an old school strong safety, I wasn't the fastest guy on the field and I liked it when it rained because it slowed up those faster than Bolt receivers out there. They had to slow their ass down, the faster they were the faster they'd slip down. You have to slow down or slip down offense and defense
Why do you think some teams water down the field (accidently of course) if they have a team full of jackrabbits up next.