Eccleston Park and Edge Hill are omitted on Sundays as well. Also Patricroft on the Manchester line.
The one hourly train on the Wigan line runs through to Blackpool so it's probably a compromise on journey time and stops, to minimise the number of units needed.
It's a tedious journey if you have to stop everywhere.
Maybe things will change with electrification (or not).
If you've not noticed, there are no trains on Sundays via Huyton at all at the moment, because of electrification works.
They seem to reappear in the May timetable change.