It's been mentioned in passing but not in much depth. It's not that Kirkby is end of the line that's the problem, it's because of physical constraints that reduce the efficiency of the station.
I don't know how well you know the area so here is a photograph of the station:
There is only one track and one platform to Liverpool (there is another platform that has hourly services to Wigan but isn't expected to have much demand even with the stadium).
That's the Wigan line ^ (as you can see there are currently two seperate lines so it's not possible for a train direct from Wigan to Liverpool Central)
The single track travels to over a purpose built bridge over the motorway and Fazakerly to station where it splits into two tracks . As it stands the frequency of the trains can not be increased any more than 15 mins apart otherwise the train would crash into a train travelling in the opposite direction as there is only track. Apparently the cost of adding a second track and building a new bridge/extending the existing one over the motorway is prohibitively expensive that can't be justified for a stadium that will be in use 26 days a year - Merseytram is a different story, if the development goes ahead it's business case is helped enormously by the presence of the retail park but it only has a small capacity for it to be considered useful for transporting supporters (from memory it was around 4,000 per hour so between tram and rail have the capacity for c. 8,000 fans in an hour for a 50,000 stadium).
There are also concerns about the loading times of passengers, i.e. there is a x (from memory I think it's 8) minutes window to load 700+ passengers which apparently is quite tight (average load time of one and a half seconds per passenger). There are also plans to extend the line to a new station in Kirkby ( http://en.wikipedia.org/wiki/Headbolt_Lane_railway_station ) which would reduce the window further making it much harder to meet the targets. Headbolt Railway station would have two tracks that would merge into one at Kirkby so that a train could wait at the station whilst another is loaded/unloaded). The proposed station doesn't help the stadium's transport case although its car park has been listed as potential park and walk site.
Pros:
Kirkby station will receive an upgrade that will allow a larger number of passengers to use the station (this hasn't been finalised yet, its things like widening platforms to allow more people to stand on it etc.)
The number of train carriages will double increasing station capacity from 1900 to 3800 per hour
Cons:
Kirkby is already one of the busiest stations on the Merseyrail network because of long commute times between Kirkby and Liverpool (40+ minutes by bus, 18 by train, 25 by car - there's a low car ownership in Kirkby (49% of households have one according to 2001 census) ).
Supporters will be held in a queuing reservoir outside the station because of the low capacity (in comparison to Goodison/Kirkdale)
Less trains are able to visit Kirkby due to restrictions of rail track (there is only one line)
This is possibly the answer to your question, spectator travel arrangements to the stadium for evening games will clash with rush hour and as Kirkby is one of the busiest stations in Merseyside there will be a lot of people trying to get on the train (a couple thousand supporters plus commuters).
Longer waiting times for trains meaning getting home later after games