Because definitions are hard. I have heard some people characterize the Russian Revolution as a coup (the argument often goes something like, well the Bolsheviks took power, no one wanted that, and they ran things pretty much the same as the Tsar; often there is the implication that the Provisional Government was actually revolutionary). By contrast, others seem to argue that the Bolshevik Revolution had mass support, particularly from their base (the working class), and that it lead to fundamental changes in society.
I am obviously not sure if either of those is a good definition of a coup or a revolution and so would be interested in hearing people argue against those definitions as well.
The Russian revolution was not a "revolution" in the majority of the population rising up to overthrow the unpopular Tzar. Some dozen Red Guards (communist party militia) took control of the Palace in St. Petersburg, overthrowing the government and placing communist leaders in power. It was more akin to a coup, as large parts of the population supported the Communists, but only a few hundred people were directly involved in the actual takeover. The real "revolution" came in the civil war, where communists fough Polish rebels/freedom fighters, Ukrainian farmers/anarchists, various political movements and forces loyal to the former government (the Whites), including the Czech Legion in Siberia.
So the actual takeover was a coup, but with mass support, leading to a revolution where the worker's rose in support of the communists to take power.