There are a few speculations as to why the series went off air. It is said that NBC is giving room to the new shows and the ratings are at a stand still. NBCs executive producer issued a statement and did not give a reason as to why citing that he does not want to be held responsible over the issue.
Because USA network doesn't know what's good! "Royal Pains" suck and yet they are staying on. Life isn't fair. I'm glad that the ending at least left me feeling a little good. May those two detectives continue to kick bad guy ass!