Mind you this is a excellent/good/fair/poor poll not an approval rating poll. So hes probably actually in the 30s as far as approval gos. Which still sucks of course.
Still what has done to derseve such approvals? He seems inoffensive and quiet compared other unpopular Governors like Scott or Malloy.
I'm not that familiar with his term as governor, but my guess is that he has no natural base to support him. Republicans aren't fond of him because he "abandoned the party" and is a liberal and a lot of issues, and democrats don't like him because he used to be a Republican and from what little research I have done it looks like he made major (in my opinion necessary) cuts to the budget that may anger democrats. In addition his fight with the legislature or members of the legislature (again not intimately familiar with the issue) in December over calling the government "Christmas Tree" a "Holiday Tree" may not have gone over well in an largely Catholic state.
*Just a side note- the latest polls on Malloy show him with a +5 approval rating (50 approve- 45 disapprove- Rasmussen Reports)