How come precarity caused the October Revolution in Russia, but it caused Fascism in Germany and Italy?

There's a very simple reason (but it's not the only reason): A significant section of the Russian military was supportive of the Revolution. Whereas in Italy and Germany, the military was by and large in favor of the fascists, by the time they were ready to ascend to power. So, in Russia, the military facilitated the Revolution, whereas in Europe, the military crushed the Revolutions.

Especially in Germany, where the loss during WW1 was attributed largely to the Social Democrats (who were pushing for an armistice), and a poor leadership ranging from the monarch to the government, as well as the nobility and the higher echelons of the officer corps. Put simply, the military hated the "leftists" and didn't really trust any other politicians except Hitler with his messages of strength, unity and revenge.

In Italy, the Vatican gradually grew in favor of Mussolini (which ended up resulting in the Vatican's independence). Virtually every priest became a propaganda officer for fascism. As a result, the military was in support of Mussolini early on.

And a second reason to consider (again, there's more reasons): The October Revolution shook things up in the rest of Europe. German communists were also getting ready to seize power, and it was clear things were about to happen in France, Spain, Italy, Poland, Romania, Bulgaria, Hungary and Greece. The capitalists and reactionaries in those countries coalesced to stop the communists before they could begin their own revolts, and they saw fascists as the natural solution. Especially in Italy, where the industrialists started heavily funding Mussolini almost immediately after he formed his movement.

There's many other reasons, like the popular mentality and culture, the nihilism of Western European communists after the failures of the Paris Commune and the more recent Spartacist Revolt, the more developed capitalist forces that permeated every facet of government and security, the large labor aristocracy that inhabited the major population centers in those countries, etc.