It is squeezed in between two VT services that do stop at Lancaster (1055 and 1108), with a TPE at 1114, so it probably wouldn't fit into the path if it did stop at Lancaster.
The southbound doesn't stop at Lancaster either:
http://www.realtimetrains.co.uk/train/Y22406/2016/10/22/advanced
...which begs the question why they bother running the unit Oxenholme-Preston-Oxenholme at all.
Presumably they run it because it's not a new service. According to my old timetables, when TPE ran the service, there was a Preston-Windermere train at about that time (which did stop at Lancaster), so if they only ran the train from Oxenholme, that would be a direct loss of a service. (Also, I'm not sure whether the terms of the franchise would require a certain number of services from Preston anyway?).
I'm sure you're correct that it doesn't stop at Lancaster because of pathing difficulties (in other words, now it's not being run by a Class 185, it can't get enough speed to have enough time to stop). But not stopping at Lancaster is presumably the lesser of two evils. At least, running from Preston, it gives a direct service to the Lakes for people at Preston, and a service that requires only 1 change to people from Liverpool, Blackpool, Blackburn, etc.
On a related note, I wonder how passenger numbers on the Windermere branch are faring now? I seem to recall reading somewhere that numbers went up quite a bit when the 185s got introduced. So it's plausible that losing them and replacing with much less pleasant trains could see some people stop using the service.