The most comprehensive 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.
This response highlights the internal divisions among the White Army forces and how their lack of cohesion impacted their ability to effectively challenge the Red Army. While other factors did contribute to the Red Army's success, such as the use of the Red Terror and external support, the disunity of the White Army played a crucial role in their defeat.