Howrah has so many Trains to Mumbai via Nagpur from SER but there is one and only ER zone train to Mumbai via Allahabad so if it originates from Sealdah,Sealdah will get one more train to serve Kolkata and Mumbai.As there is also two train from Howrah and Shalimar to LTT,Sealdah becomes more popular if there will a daily train service to Mumbai besides New Delhi :)