Hi MUFC_fan,
During the Winter, northerly cities in North America experience a surge in vacation traffic to Florida, Las Vegas and the Caribbean (similar to that experienced from Britain to the Mediterranean in Summer). Customers from cities such as Boston and Toronto wish to "escape to the sun" during the bitterly cold months. Consequently, it is financially lucrative for airlines to redeploy equipment to these markets when demand is at its peak, and to suspend or reduce service from 'low-season' markets to make way.
Airline operations are all about the pursuit of profit. During Winter, routes such as BOS-MIA and YYZ-LAS are heavily booked at strong yields whilst demand for MAN-YYZ and MAN-BOS is muted (even if an aircraft is full low-season yields can be poor). In Summer the reverse is true. Therefore, even if MAN is capable of supporting flights to YYZ and BOS in the Winter, the scarce aircraft resources can be more profitably deployed elsewhere. The most profitable option will be chosen by the operator.
It is also for this reason that you will notice that many charter aircraft are based in the UK May to October and Canada November to April (they will all be home to roost at MAN again soon!). North America and Western Europe simply have peaks of vacation demand which are diametrically opposed to each other. This represents a financial opportunity to airline companies.
Seasonality is the reason why MAN loses many services in the Winter (including the AAL Boston and the ACA Toronto). But equally, one could argue that we GAIN them in the Summer at the expense of leisure routes serving the North American vacation market. The aircraft simply fly where the customers are most willing to spend their money at the time.
I hope this explanation is helpful to you.
All the best, SHED.