Unbelievable that so many of you believe the juicy media 'blame the pilots, story'.
Maybe the gearpins were not removed before departure. This should be checked by the crew but apparently they didn't. Realizing their mistake directly after takeoff, they have a few options. Usually you would land at the place of departure, stow the gear pins, fuel up and buy a round of beer at your next layover.
However, maybe they were overweight for landing and preferred to proceed in the right direction while burning fuel?
Maybe they called the company and asked for advice. Maybe Kolkata was very congested and the company told them to continue towards destination and make a fuel- and gearpin removal stop along the way? Maybe Nagpur is a better place for maintenance?
Maybe this crew didn't want to admit the screw up because of cultural blaming/shaming issues and they preferred to continue towards destination?
In short: we don't know the full details here and it's not clear that the pilots are the ones to be blamed for this.