Why was force necassary to get grain from the peasants during world war communism?

Because why would a peasant willingly give up his/her grain?

Here it is in a nutshell:

Look up agricultural collectivization and kulaks while you're at it. Stalin wanted to industrialize so one of the reasons he enslaved the peasants/land was to pay for industrialization. The money raised from the seized grain of the state farms paid for industrialization by being sold on the international market. Industrializing the country stalin claimed would benefit the soviet union.