The reasons I always state in these threads;
1. The Whites lacked popular support. Without the peasants they can't win.
2. The White Generals could not cooperate
3. The Whites did not recognize breakaway nations like Poland, Finland, the Baltic Nations etc. They gained no support from Russia's neighbors. Instead the new nations conisidered the Whites as much an enemy as the Reds.
4. The double edged sword of foreign intervention. The allied powers had no interests in doing anything useful in Russia, and only gave the Whites a bad name for being foreigners.