Sealdah is mainly occupied by local and Assam/North Bengal bound trains. Also trains from south have to crawl over the whole Kolkata city to reach upto SDAH which includes crossing the river, same goes for Chitpur also.
Howrah, Santragachi and Shalimar are the most feasible points for South bound trains, however Shalimar is least preferred option for catching a train due to its poor connectivity and Santragachi also has some terminal limitations. Hence railways has no choice but to knock the platforms of HWH.