This is a very good question! I consider myself decent at understanding military history but i have also frequently marveled at this myself. It is such an incredible political and military achievement that (along with the Chinese and the Cuban revolutions) this question really deserves to be studied in depth by any revolutionary movement that truly wants to win. My own understanding only scratches the surface so i look forward to reading what other comrades have to say and looking into book recommendations as well.
One thing that comes to mind that i have heard was kind of a "secret weapon" of the Bolsheviks was their control over the transportation network of Russia, in particular the railway system (with a majority of railway workers being solid Bolshevik supporters despite the old union leadership being more inclined toward the Mensheviks/SRs) that allowed them to deploy their logistics much more efficiently than their enemies over the vast distances of Russia.
They also learned very quickly how to efficiently organize and mobilize large numbers of people (though what they were able to achieve at this time pales in comparison to the truly monumental scale and efficiency of mobilization that was achieved some decades later in the Great Patriotic War, which to my knowledge is unsurpassed in history).
As well the revolutionary fervor they were able to inspire gave them a big morale advantage over their enemies and allowed them to absorb temporary defeats and losses that might have unraveled other armies.
Finally, unlike the other various anarchist and non-Bolshevik social revolutionary groups, the Bolsheviks understood the necessity of discipline (learned over years of political struggle prior to the revolution).