Interesting one this. On the face of it, having the Wigan lines pass immediately behind the platforms but not have their own is totally illogical. However, as in so many other cases on the rail network, I think this is an accident of history - although relatively recent history in this instance.
Prior to closure in 1966, Lostock Junction station had platforms on both the Chorley and Wigan lines, situated in the V of the junction. The present station opened in 1988 as Lostock Parkway (incidentally, when did the 'Parkway' bit disappear?) In common with most station (re-)openings at that time, it was done as cheaply as possible. However, at that time Lostock Junction signal box was still in operation, located next to the junction on the 'Up' side (towards Bolton), and the Up Relief started just to the east of the Signal Box. This meant that the platforms could not be built to the east of the junction, thereby serving both routes. Since the station re-opening was of the 'suck-it-and-see' variety (that is to say it wasn't known how successful the station was going to be), the decision was taken to just open platforms on the Preston line, since the Bolton to Wigan line only had an hourly service and it was thought that the Preston/Blackpool service would be more likely to bring in the punters.
Just two years later the Signal Box was closed and demolished, and the Up Relief removed (along with the Down Relief further east.) In a classic example of the joined-up planning that we do so well in this country, if the station re-opening had been delayed by two years, it could have been built on the Bolton side of the junction and served both routes. Whether GMPTE knew this was going to be the case and chose to go-ahead anyway, or whether BR just didn't tell them, (or indeed whether it was a conscious and deliberate decision) I couldn't say. Given that the remodelling was part of an on-going process to bring the area under the control of Manchester Piccadilly Signalling Centre, I'm assuming that BR's plans were well-advanced when plans for the station were made.
Since 2004/5, use of the station has increased by 127% to 268,000, so it seems likely that patronage would increase still further by addition of services to Wigan and/or Southport (and of course a concomitant increase in services to Bolton and Manchester.)
However, as others have said, I believe there are issues with the signalling overlaps on the Wigan lines which would require attention before platforms could be opened. Moving the station further east beyond the junction is an expensive option, and would be a major constraint on capacity on a busy stretch of two-track railway. Since this is the case, opening platforms on the Wigan line would make perfect sense though. It would allow for an increase in service at the station without stopping trains blocking the Bolton - Preston main line. According to Wiki (take that as you will!), Network Rail's North West Route Utilisation Strategy included provision of the two extra platforms, but that was before electrification to Wigan North Western was announced - and of course it was dependent on funding being found from somewhere!