This is a complicated question. Basically, the 1905 Revolution really got going as the Russo-Japanese War was ending and troops were returning from the front. These troops, most of whom were either rural (enlisted men) or nobles (officers) had little sympathy for the mainly urban 1905 revolt. The 1905 revolution thus found little support from the military, and when it came down to fighting in the streets, they were easily defeated.

1917 found Russia in the midst of a war that looked like it might drag on for more unsuccessful years. Russia had essentially lost the war at Tannenburg in August 1914 - the next 3 years were simply prolonging the inevitable. Soldiers were more widely drawn from the population due to the size of the war, and Russia had urbanized signifcantly since 1905. Thus when revolt broke out in Petrograd, soldiers rapidly defected to the revolution in due to their promise of peace.

This is a very brief answer. If you'd like to know more, I'd highly recommend Orlando Figes A People's Tragedy, which covers this period in Russian history in great detail.