I believe they did dominate the Soviets along with the Left-SRs, the problem was their support of the war compromised them, while Lenin and his tiny band of Bolsheviks grew in favour by calling for peace at any cost. Get rid of Lenin (say the Germans don't let him through to Russia), and the Bolsheviks will be a lot less coordinated and effective, possibly butterflying the October Revolution away.
However although the Mensheviks had more popularity, they're still an urban Marxist party, they're never going to dominate the electoral polls unless in alliance with the pro-peasant SRs is solidified, and even then they'll be the leftwing fringe of the alliance. These might be pretty crude parallels but if the Russian Republic can remain, I wouldn't be surprised to see the SRs dominating as a broadly social-democratic force, ala the Indian National Congress, while the Mensheviks would form the radical (though not revolutionary) fringe much like the Indepedent Labour Party in Britain and the Independent Socialists in Germany.