Last summer i got into some wicked turbulence (for me). It was so bad there was no way I could use my touch screen gps while controlling the aircraft. I wanted to get out of this at the closest airport but no way could i get the freq's up on the gps. I ended up flying another 20 miles to an airport that i knew the freqs.
Lesson learned - I now print out kneeboard data for all airports along my route of flight.
Touchscreens are great - except in significant turbulence in light aircraft.
Even in light turbulence and chop the touch screens get difficult to use, especially if IFR where a course or altitude deviation gets more serious. Its one of the reasons why I hate the move to touchscreen in the cockpit. I understand it allows for a better space usage with a larger screen and more bells and whistles/options on the GPS but you very quickly sacrifice usability under certain conditions (which also includes flying with gloves such as in cold weather).
Add to that dirty fingerprints on the screen and feature bloat that no one really uses or understands (but may be valid questions on a checkride)... There's other reasons too but those are the 3 big ones that come to mind.