Step 1: Start with the raw material.
West Bengal's fertile Ganga-Brahmaputra delta and humid climate are ideal for growing jute, so mills there get raw material without high transport costs.
Step 2: Add water and labour.
The Hooghly River supplies the water needed to process the fibre and also serves as a transport route, while the region offers cheap labour in large numbers.
Step 3: Add infrastructure and history.
Well developed roads, railways and the Kolkata Port made trade easy, and since Kolkata was the capital during British rule, entrepreneurs already had banking, finance and export networks in place there.
All these factors together explain why most jute mills are concentrated in West Bengal.