Soumya Sen asks why CPM keep winning in Bengal. And he concludes it with the reason as there is no good alternative.

In fact Calcutta went to become the first city to support a huge gay rally on the streets, effectively laughing in the face of the antiquated Indian Penal Code that describes same-sex relation as illegal. So BJP and Shiv Sena simply do not have the right cards to play in Bengal. Caste-based politics is also absent in Bengal (thankfully!) and so Mayawati and Karunanidhi also have the wrong cards. And Rajnikanth’s on-screen antics can only turn away the voters in Calcutta.

He also cites another reason, which I think is true about Kerala’s political scenario. The CPM in Kerala is not gaining votes for what they are now, but for what they were.

In rural Bengal CPM is probably popular because of the land-reform and redistribution initiatives etc.