Why is ditching speed higher than stalling speed?
As far as I'm aware - and maybe a seaplane pilot could give a better answer - one of the big issues with ditching is judging your height above the water. If you stall it in, you might get it wrong and lose control completely.

I checked the POH for a 182RG, which has a Vso of 39 KIAS and ditching speed of 60 KIAS. The ditching checklist gives descent rate as the primary consideration, not airspeed:


  
  Power -- ESTABLISH 300FT/MIN DESCENT at 60 KIAS
  


In the amplified procedures section, it says:


  Avoid a landing flare because of difficulty in judging height over a
  water surface


In other words, better to descend slowly under control until impact than stall it at an unknown height above the water. As Bob Hoover famously said, "If you're faced with a forced landing, fly the thing as far into the crash as possible".

I did a bit of Googling and found similar checklists for other aircraft, e.g. a Piper Arrow:


  
  Power--ESTABLISH 300 FPM DESCENT AT 90 MPH