I have seen that bengali brahmins are non-vegetarians whereas their counterpart in the rest of India are strictly lacto-vegetarians.
Can you tell me why this difference in their culture?
Not all Bengali Brahmins eat fish. Vaidikas, who migrated from Kannauj, follow the same dietary rules as Brahmins in the rest of the country.
The reason for the relaxation of rules among the indigenous brahmins was Kulinism, introduced by King Ballal Sena (1160–1179). This soon led to polygamy, where women of lower castes could be married, which eventually diluted the strict caste rules followed elsewhere.