A number of reasons. A couple of big ones:
The White Army was made up of the Bolshevik's enemies (namely Tsarist-Imperialists, Socialist Revolutionaries, and even perhaps Mensheviks), which formed a type of coalition. They were for one, highly disunited. Not only this, but they were fighting against a very well organized Red Army (in part organized by Trotsky, who was very brilliant) with a high morale and many capable commanders. This was in my opinion one of the biggest problems; the Whites were very often in opposition between themselves. Not a good thing when you're trying to unite and win a war.
The White's other big issue was that their armies were separated by long distances. The Red Army was able to easily take them one by one. Trotsky once again was a brilliant leader and exceptional at organizing, so he was able to capitalize on these weaknesses.
They also didn't have the economy on their side. They were trying to create a stable Russian republic and use its economical power to overthrow the Bolsheviks, but it was too late.
Basically, I'd say it was due to a lack of formation and unification within in the White Army.
The simple answer is that there was no unified opposition to the Bolsheviks. The corollary that the Bolsheviks were very well organized is sometimes overstated, but there is some truth to it.
Certainly there was a great deal of resistance and opposition to the Bolsheviks, but it was disparate and unorganized. The so-called 'Whites' were counter-revolutionaries through and through, and represented a narrow sector of landlords and businessmen who opposed the Bolsheviks on fundamental grounds. They were best represented by two armies: one in the south led by Anton Denikin and one in the east led by Alexander Kolchak. As I see it, there were a few problems with these movements. First, they had virtually no popular support and did little to help that. They had no effective propaganda, and often viewed SRs and Mensheviks as identical to Bolsheviks for political purposes (this was particularly true in Siberia under Kolchak). Second, their armies were made largely of Cossacks who had more narrow local interests, particularly in the South. For example, in the Don and Kuban regions, the population was roughly split equally between Cossack and non-Cossack, but the Cossacks controlled the most valuable and productive land, giving them considerable economic privileges. Many non-Cossacks were forced off land and into the growing industrial centers in Rostov and Taganrog, and later many became Bolsheviks. The conflict between Bolshevik and 'White' in the South was, thus, mainly a conflict about the political-economy of the South. Many Cossacks had little concern about Moscow in comparison.
Various socialist groups also opposed the Bolsheviks after January 1918 and the dissolution of the Constituent Assembly. Several processes of the Civil War period frustrated their efforts. First, they had no capacity to ally with the Whites, even if they wanted to. In fact, the SRs established their own socialist state in Ufa in 1918, but it was eventually overthrown in a military coup led by Kolchak. Second, SRs and Mensheviks had difficulty cooperating anyway because of basic ideological differences. Third, the general economic collapse of Russia at the end of the Great War - a major cause of the revolution in the first place - had continued and drove depopulation of cities as people fled to the countryside looking for food. This caused a social erosion of the industrial working class and the popular elements of the urban revolution, allowing for greater concentration in the hands of the Bolsheviks and a weakening of the Menshevik opposition. Finally, in the war period the Bolsheviks began to use the Cheka to repress various uprisings, mostly related to food shortages rather than specific ideological programs. The Cheka reported, for example, 245 uprisings in 1918 alone. This would have facilitated the growth of the Bolsheviks' capacity for political policing, which was used extensively against socialists as well as other opposition forces during the Civil War period.
Finally, various nationalist forces also resisted the Bolsheviks and used the collapse of the tsarist regime and the continued general collapse of Russian society as an opportunity to break away. In some cases these efforts were successful, such as with Poland, Lithuania, and Finland, and in others it was not - most notably in Ukraine. These groups were not interested in overthrowing the revolution in Moscow, but in securing independence from their age-long domination by Russia.
The success of the Bolsheviks also partly reflected the changes in early Soviet society from just late 1917 to 1919. The Bolsheviks had an on-going conflict between the center leaders and provincial leaders - a conflict which historians now maintain characterized much of the early USSR period in general. To be fair this wasn't a simple matter of the evil Bolsheviks in Moscow repressing the more democratic local Bolsheviks. In many cases, it was just the opposite - the local officials were more brutal than the center, who wanted to be more conciliatory for political reasons with opponents. Nevertheless, this conflict and the emerging fact that 'localism' became a buzzword of political significance in Moscow political circles led to a centralization of the party apparatus in many of the key areas near Moscow. Along with this centralization came the various political tools at the party's disposal: propaganda, organization, terror. This certainly allowed for a more unified line and more unified enforcement of policy than any opposition group could.
Sources:
Read, Christopher. From Tsars to Soviets: The Russian people and their revolution, 1917-21. London: Routledge, 1996.
Stewart, George. The White Armies of Russia: a Chronicle of Counter-Revolution and Allied Intervention. New York: Russell & Russell, 1933.
Kenez, Peter. Civil War in South Russia, 1918: the First Year of the Volunteer Army. Berkeley: University of California Press, 1971.
Mawdsley, Evan. Russian Civil War. Boston: Allen & Unwin, 1987.
Smele, Jon. Civil war in Siberia: The anti-Bolshevik Government of Admiral Kolchak, 1918-1920. Cambridge: Cambridge University Press, 1996.
Swain, Geoffrey. The Origins of the Russian Civil War. New York: Longman, 1996.
Brovkin, Vladimir. The Mensheviks After October: Socialist Opposition and the Rise of the Bolshevik Dictatorship. Ithaca: Cornell University Press, 1987.
Raleigh, Donald J. Experiencing Russia's Civil War: Politics, Society, and Revolutionary Culture in Saratov, 1917-1922. Princeton: Princeton University Press, 2002.
Since no one has answered I'll try my best with limited knowledge.
Firstly, the Bolsheviks Who we call the reds, were a united group rather then constructed of various loosely affiliated parties which made up the white armies. The whites were derived from liberal groups within Russia such as supporters of the Kadets, some were foreign armies sent by Britain, America and many other countries, while on the other hand, some groups were staunch royalists who wanted the Tsar back in power either as an autocracy or constitutional monarchy. So of course having many diverse parties that had widely different motives made it more difficult to fight as a united front.
Secondly, the Bolsheviks were in control of all the major industrial centres such as Petrograd and thus had a strong hold on heavy industry output and supplies for the troops. Further more the Bolsheviks also had a firm grip on the transportation system which limited the white armies ability to move troops swiftly around Russia,
My knowledge is a bit hazy but I hope that helps for now !