I suspect part of this is because of the bad timings with the long wait at Oxenholme - it can be quicker to take a following service and change.
I've observed similar of the Barrow, that said - when I've used it it's either been locals going to/from Lancaster or connections much more to London than Manchester, and then an almost entirely separate Preston-Manchester fast. With well-timed connections to/from the Scottish TPEs (this is the UK, right) you could get away with not running these to Manchester at all; 3tph Preston<->Lancaster (two Avantis in normal times and a TPE) is still decent.
I have wondered about the idea of Lancaster P1/2 being turned into a "Lakes and Furness Lines" ministation with it all based there with dedicated, branded units, a bit like the way the S&C and "Hadrian's Wall Line" bit at Carlisle.