Politico's story sort of hints at what his true motivations might have been...[
But I think the real story here is probably just that there's very little advantage to being in party leadership in the Senate. It is true that party polarization has increased in the Senate as it has elsewhere, but that hasn't really meant that party leaders are particularly powerful; in the Senate as always, it's individual Senators who matter. Formal position, whether it's in the leadership or on a committee, matters...but just not all that much. Obviously the status or the influence or whatever is enough that the jobs don't go unfilled, but they just aren't remotely similar to House party leadership.
*Corrected; I had this wrong originally. He's up in 2014.