The rumor I heard is that Holder wants to stay until after the celebrations for the 50th anniversary of the Civil Rights Act this year.
It will be easier for the Dems to get a radical replacement confirmed in the Senate this year than it will be after the election, or in the runup to 2016.
Holder has been there a long time already. He may be tired and want to cash in, or they may need to change him out because of his baggage. Whatever the reason, indications are that the Senate will be less favorable to the Dems after the election - maybe even a real obstacle.