The best explanation for why the Red Army won the Russian Civil War is:
Although many groups opposed the Bolsheviks, the White Army was not very unified and this made it harder to have success in the war.
While factors such as the effectiveness of the Red Army, the use of Red Terror, and limited support for the White Army from outside countries played roles, the lack of unity and coordination among the various factions of the White Army significantly hindered their chances of success against the more organized Red Army.