Just copied and pasted this from my notes last year so hopefully this helps... I know it's more than one reason but you can form a judgement on the main reason. Sorry that there's a fair bit but hopefully it's useful
Stalin’s motives for collectivisation.(Policy of creating larger agricultural units where the peasants would farm collectively rather than on individual farms).
*Control the peasants as had grown wary of government so grown less food, rather than grain requisitioning ‘farm requisitioning’.
*Promoted co-operation and common ownership, core values of socialism.
*Link with industry:
-Fear of invasion from foreign powers (Germany) = urgent need to industrialise= only possible if supported by an increase in agricultural productivity (army needs feeding at war!)
-New industries could also require technology from abroad so food surplus was needed to get foreign exchange.
-Labour also needed in new industrial centres =achieved in short term by mechanisation of agriculture= labourers released from countryside to work in industry.
* Economic factors:
-Agriculture was very inefficient compared with the rest of Europe.
-Resulting larger units to make the use of machinery more viable and cost-effective=food production increased=reduce labour requirements of agricultural production= release many much-needed workers.
*Political factors:
-Help to extend socialism to countryside/ get rid of remaining elements of capitalism = ensure survival of revolution.
-Opportunity to get rid of the kulaks (richer people who benefited from NEP).
*Allowed Stalin to rid the party leadership of the right who had been in favour of NEP=
aimed at increasing his own position and power.*Eliminated threats from inside and outside the party.
What was the case for rapid industrialisation? (implemented through the five year plans)*Believed that the revolutionary transformation of society would only be achieved through industrialisation, (Marxist ideology).
*Majority of the population were peasants= only had a limited commitment to the regime- not proletariat!
*Disillusionment felt by many in the Party and working class towards the NEP.
*To harness the enthusiasm of many workers= build support.
*Fear of war- anti-Communist governments in Finland, Romania and Iran, both Britain (1927) and the right-wing polish government (1926) broke off relations with the USSR , Hitler’s anti-communist statements and in the 1920’s in was still producing less coal and oil than France= sense of isolation and vulnerability and industrialisation was seen as patriotic.
*Only hope to resisting a potential attack was through improving industrial capacity and strengthening the USSR= deter aggression and repel it if deterrence failed.
*To wrong-foot Stalin’s opponents- left Trotsky and Zinoviev with little that was distinctive about their beliefs (Stalin stole their idea?) and isolated Bukharin.
*By achieving industrialisation Stalin’s importance would also be comparable to that of Lenin.
*Stalin was responding from a strong pressure within the party as they saw it as a necessary development to ensure the survival of the revolution= would create more proletariats and get rid of Nepmen
*Would enable the government to direct the economy and ensure the adequate production and distribution of essential materials including the food needed to support industrial and urban growth= Russia’s economic resources maximised.
*Contrast with the west-Russia’s commitment to economic growth and full employment was a stark contrast to the west (after Wall Street Crash) which seemed to justify Russia’s claim that its ideology was superior to capitalism= gained support for industrialisation.