Kolkata is one of the biggest trading hubs in entire south Asia because it is the biggest and busiest city in and around a dozen Indian states (and some foreign countries including yours), all connected with railways, highways, airways, riverways and seaways, it works as a hub for a humongous mass of land. And all the cargo don't need to go through Kolkata or Haldia ports, we also have a couple of deep sea ports around at Paradip and Visakhapatnam with cargo handling figures of 68 and 58.50 million tonnes respectively for 2013-14. We have options, unlike Bangladesh.
And about your Farakka Barrage comment, if you have multiple businesses, then it doesn't mean that you will discard all but the biggest one.
Chittagong is a port city, and Kolkata is a major city of India that also has a port, there is a difference.