Dr.Bidhan Chandra Ray perhaps was the best CM not only in Bengal but in entire India. But after his death nobody was their the hold the bastions for Congress. During Siddhartha Shankar Ray's rule,especially during emergency Bengal merged into one of it's darkest phases in it's history. The massive state oppression on the Naxals had adverse effects on the educated Bengali intellectuals who slowly drifted away from the Congressis. This is more or less the reason the Commies came to power. They solidified their position by a huge program of land reforms in rural Bengal. The remaining feudal system got completely abolished and the communists exploited this for the last thirty years.
One thing they mastered is in their propaganda mechanism and the rigging machinery. Like their Stalinist counterpart in Russia and Maoist China, they too understood the importance of the above two factors to sustain their stay in power for a long period.Slowly they buy out their main opposition, the Congress. Their other left allies never got a chance to swell up and the party cadre mechanism spread every school, government office and factories. This is the reason it took 34 long years to get them uprooted.