Probably down to two reasons. Firstly, they were relatively indecisive, and they didn't want millions of soldiers returning home to an already unstable country, they feared revolution. They were not as radical in their policy as the Bolsheviks, nor did they have the relevant backing of the Duma to make such a move, democracy, aye? Secondly, they wanted to keep a good level of relations with the Western Powers, so leaving the war was not an option.