Most of the Lapland charters leave at breakfast time and get home late evening. And are sold as a children-and-family day out.
We can be certain that when the aircraft returned to Manchester it would be late evening, that the buses would not have even been organised yet, they would doubtless not have got away from Manchester until well after midnight (it being not an easy job to find multiple bus drivers for a 10-hour round trip on spec at 10 pm at night) and with the misty conditions on the motorway would have turned up at Gatwick round about breakfast-time.
And this was an operational decision. The aircraft was not tech and the crew were in hours to go to Gatwick.
I can only presume the commonsense team at Monarch ops were not on duty that evening.