Because the SR didn't start the Communist revolution in Russia.

There were two Russian Revolutions in 1917, the February revolution caused the abdication of of the Czar was led by a broad spectrum of political factions in Russia, the SR prominent among them. This phase of the revolution was not Communist in character and ended with the establishment of a provisional government. At this point the Bolsheviks were a fringe far left party, with membership in (at very best) mere tens of thousands in an empire of over 162 million people.

The October Revolution however, was led by the Bolsheviks against the provisional government headed by Alexander Kerensky who was part of a "right-wing" SR faction. The Bolsheviks had greatly increased their strength and membership in the month following the February Revolution. Many "left-wing" members of the SR defected to the Bolsheviks but there was no doubt that Lenin and other Bolsheviks were in the driver's seat during the October Revolution. The Bolshevik's base of support came prominently from factory workers in St.Petersburg and Moscow, whereas the SR enjoyed more support in the countryside where the majority of Russia's population lived, but the support of the factory workers were much more important to winning the struggle in the capital. This was the revolution that led to the establishment of a Communist government in the capital and eventually, after a long civil war: most of the former Tsarist Empire.

The Social Revolutionary Party, like the Social Democratic party, was divided into two factions: a revolutionary Left wing, and a status quo right wing. The respective factions of the Social-Democratic party are better known as the Mensheviks and Bolsheviks.

The October 1917 revolution was not just against the old order, but also against the right-wing socialists. Kerensky, the head of the Provisional Government, was a Social Revolutionary(from the right faction) as were several of the ministers. The Left SRs supported the Bolsheviks, and in some cases were even further to the left.