It is harder to find a good hitting 2nd baseman than it is to find a good hitting 3rd baseman.
Therefore if you have a guy with a good bat and he could play both 2nd and 3rd, it is much more advantageous to have him play 2nd full time as a good hitting 3rd baseman is a lot easier to find.
Just think about how many good hitting 3rd baseman there are in baseball and compare it to the good hitting 2nd baseman (Cano, Utley, Pedroia and Kipnis).