Newer single engine cessnas only go to 30 degrees because it's way too hard to power out of a rejected landing with 40 degrees in certain conditions. I can fly that old C182 with 40 degrees at some pretty crazy slow speeds given it's weight and size, but with that much induced drag you just can't maintain altitude at all. With these aircraft there was also the issue of the flaps blanking out the tail when speeds decrease during landing, resulting in flat/hard landings as well. Limiting to 30 degrees helps with both. I don't believe it had anything to do with slips.