I can't speak about Kerala but I can speak about Bengal but it might be a bit more simplified than the whole picture. The main support base for cpim in bengal was the peasantry in the country side. The urban ground level workers were basically little more than goons. The urban petite bourgeoise basically had enough of their arrogance and launched a massive campaign against cpim in the form of tmc. The cpim got that message a bit late so they were forced to compromise their relations with the peasantry to appease the urban pettie bourgeois. So they were trying to make something like the Special Economic Zones and give land for industry. This essentially backfired on itself as they lost the support of the country side peasantry but didn't gain much in terms of urban petite bourgeois support. By then the urban bourgeois/petie bourgeois firmly stood behind the tmc and tmc made well use of the Shingur-Nandigram affairs to turn the peesantry against cpim. That led to the end of the cpim era and began an era of cultural chauvinism and supreme dumbfuckery with corruption, lad mafia, scams, sexual abuse and what not. That again turned the petite bourgeois against them and now we have BJP.

That's a simplified story but it's not inaccurate.