It's not only in West Bengal. Tea was still sold in kulhars (kulhads) in North India (from say Madhya Pradesh and up north) some years ago when I used to travel

It's not only in West Bengal. Tea was still sold in kulhars (kulhads) in North India (from say Madhya Pradesh and up north) some years ago when I used to travel more in trains - this was decades after Independence in 1947. Apart from the disposable factor, I guess another reason may be that the clay pot keeps the tea warm for longer - and it can get pretty chilly in North India in winter, though not quite comparable to more northern countries. It even gets quite chilly in say Maharashtra (south of Madhya Pradesh), more in the interior areas away from the coast, in winter.