Reagents in Krapcho's decarboxylation
So as mentioned the Krapcho-Decarboxylation follows a SN2-mechanism and is therefore working best with a methylester.

The solvent choice for DMSO (polar aprotic) is due to low stabilisation of the chloride anion rendering it highly reactive for the nucleophilic attack. The resulting methyl chloride is gaseous at room temperature and leaves the reaction mixture (le Chatelier -> formation of product strongly favoured)
The leaving group of this substitution reaction decays into CO2 and the decarboxylated product. (Again: formation of a gas)

I think that the destabilisation of the cyanide in DMSO and the formation of stable acetonitrile can be considered as the driving force in the second reaction.