There are many factors why West Bengal failed to proceed in a same speed like some other Western and Southern states. I will recommend you to read two widely respected book on this subject. 1. The agony of West Bengal by Ranjit Roy and 2. The Spoil of Partition by Joya Chatterji
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West Bengal's decline has historic causes, Central govt.'s descriminatory policies and West Bengal's own vicious political culture to blame. I will highlight some-
1. Partition caused the disruption of economic activities in both East and West Bengal. Calcutta being the premier industrial and financial hub declined as it lost its source of raw materials and market of East Bengal.
2. After independence, Indian central govt. made a lot of policies which hurt economic prospects of West Bengal and hurt it's recovery from economic disruption brought by partition. Freight equalization policy and discriminatory policies to grant industrial licenses and other financial benefits hurt West Bengal and stimulated Western and Southern States.
3.Naxalism and Communism got fertile ground in West Bengal which sacred away investors and industrialists.
4. 34 years long Communist rule and politicization of economy badly hurt West Bengal's prospect as a hub of free market economy.
5. Babu culture among the elite and educated Bengalis of West Bengal. They are more content to get a govt. service jobs and settle down for low hanging fruits than taking risks and becoming intrepreuneuers and industralists. It has something to do with caste system among Hindu Bengalis where merchants and traders were not respected compared to professionals like Doctors or lawyers.
6. Mamata Bannerji's earlier short-sighted policy to force the Tata group out of Bengal when they tried to set up an Automobile factory. This high profile incident gave West Bengal a very bad reputation of not being a suitable place for business and industries among leading industrialists from India and abroad.