Monica,
I think the reason why students do not prefer IIM Calcutta has nothing to do with the quality of the Institute. It has more to do with what is called the art of packaging. IIM Calcutta had never indulged in promotional activities for itself as a brand, in the past. Now it has realized that advertising has got to do a lot with how aspirant students perceive you. The fact that Calcutta or for that matter any eastern city is less preferred by students has also played a role. Look at the placement scenario there and decide, IIM Calcutta is at par with any other IIM:
Average Indian salary - Rs. 6, 98, 800
Average foreign salary - USD 98, 321
Total no. of offers - 288
No. of foreign offers - 84