Ulhas
I wish I knew. I bet the CPI(M) wishes it knew, too. Here are some hypotheses: (1) It's better to have alternation in power than to rule for 25 years at a stretch. It keeps your feet to the fire. (2) Jyoti Basu in West Bengal invited in foreign investment, while the Left Front governments in Keral didn't. (3) Gulf remittances have allowed the governments in Kerala not to worry about the effects of high levels of government spending and a highly mobilized population on capital investment. (4) Much of Kerala was formerly composed of two princely states (Travancore and Cochin) with princes that were more progressive than the Brits were in Bengal. (5) What really distinguishes Kerala is that women have shared in the benefits of development, perhaps traceable to the influence of the matrilineal Nayar caste.
But I repeat, I'm just throwing out possibilities here. If I knew the NECESSARY conditions for performance like Kerala's, I'd be shouting them from the rooftops.