I guess before it used to terminate at New Jalpaiguri only, then they extended it to Haldibari.
Now even after extension blocking Platform for almost 1 hour 15 minutes doesn't make much sense but maybe due to high patronage from New Jalpaiguri (evident from its high number of AC Coaches), they have given it so much time to allow passengers to settle down in the train.
Plus it must be serving as a connecting train to a Toy Train coming from Darjeeling. So they must've given it this much time period to compensate...
more... for delays.
Localites may be able to give more accurate details on this issue.