The reason is actually rather more simple than that. It is now two years since McCourt signed for Celtic. Since then he has been managed by Strachan (who signed him), Mowbray (who inherited him) and Lennon (who played/trained alongside him).
In that time he has made just 5 SPL starts (none of them against Rangers btw). I'm not sure how many of those he has finished, but I wouldn't be surprised if it was none. He has also been left on the bench (or discarded entirely) for Celtic's big Cup and European games. Moreover, for all this talk about him being an "impact sub", he is invariably only gets brought on late from the bench, often the final of the three subs. This includes games such as Braga and Utrecht this season where Celtic fell behind and were desperate for a goal.
Why should this be? Quite simply, he is not fit or strong enough to play full-time professional football to a decent standard. Both Strachan and Mowbray publicly urged him to improve his fitness and I have no doubt that this only followed their telling him the same in private. Similarly NW has said that if/when he gets a run of games for Celtic, he will come back into consideration for NI.
There is no good physiological or medical reason why McCourt has not got fitter; therefore none of his managers can go back on earlier demands for him to get fit, otherwise less talented but harder-working players would either take the hump, or start slacking themselves. And that is poison to any team, never mind one like NI, where hard work and commitment are often all we have to offer.
But before anyone thinks I am in some way biased against Paddy, I can honestly say I would love nothing better than to see him in an NI shirt, turning it on and frightening defences. I just don't want then to see him turning off and frightening his own defence.