A lot of people on here seem to be asking for United to operate a longer summer schedule, or even year round. How exactly is this going to help the route be a success? If you can't fill a 757 in the busy summer months, how exactly are you going to fill it in the winter months?
Apart from a possible peak around Christmas time, there's very little demand in winter. Look at EDI for example - 5 daily flights to the US in the summer, dropping to just 1 (possibly 2) in the winter.
Year round may be better for attracting the loyalty of business passengers, but you still need the tourism traffic to be there in the summer to make the route viable.
Like every other route they have started, it's summer only to gauge demand. Once they see the demand is there, then they will try and operate a winter schedule too.