My understanding is that due to the very very late arrival of the 787, Monarch have been offered 737NGs at a very very good rate by Boeing.
Finding experienced crews, in short order, may be tricky for a large increase in fleet size; so from a business perspective it could make sense for MON to buy baby for the 737 experienced crews. It would save time and training budget, would give them some ready made bucket & spade routes and I am sure the staff would have no problem readjusting to a new owner.
Just my uninformed conjecture.