This is just a guess with regards Monarch, the 720/707 was probably a better fit for their trans Atlantic ops due to initial price, load capacity and range capability.
When Monarch started operating 737-200s and 300s around Europe, they could easily do what the 727 could with a lesser fuel burn, less maintenance and without the additional cost of a flight engineer, no disrespect meant to FEs. Then 757 put an end to the 727.