I think it's mostly because of the fact that the IWC loves to play rebel. It started in this case when Edge got pushed and the IWC thought "Hey, why not Christian as well?" At that time though, to be honest there was not much to choose between the two except for maybe the Spear on Jeff Hardy at WM 17 which I believe even Christian could have performed.
Then, I guess that the fact that he was not getting pushed was linked to the fact that he did not have the right body type. In other words, it was linked to the "Vince loves big guys" stuff which overlooked the fact that he was just not ready at that moment. If you look at his matches in 2002 and his matches today, you would see a world of difference. Even his promo cutting skills were not that good. He was funny, sure, he has always been funny, but he not serious enough to really sell across a feud.
Then he got a slight push when he feuded with Chris Jericho. He got a kickass theme music, nickname and even a bodyguard at this point and the IWC started to get even more restless about his push. He even main evented a PPV with John Cena. Some of his promos during this while were completely off the charts and even better than the promos he cuts today. Alas it was a case of good but not good enough with Christian. The roster in 2005 when all the cool things were happening to Christian, was loaded on both brands from top to bottom. As much as some fans like Christian, I am sure they wouldn't rate him as someone better than Angle, HBK, Jericho, Eddie Guerrero, Rey Mysterio, Undertaker and Triple H. You also had a fresh into the main event Cena who was getting a huge positive reaction at this time and Batista in the hottest phase of his career as a face. That left no room for Christian.
And to complete his love affair with the IWC, Christian went off to TNA. That gave the IWC one more insinuation to level against Vince as the reason why he was not pushing Christian. Also there is this rumor that Christian was supposed to cost Jeff Hardy his championship and have a match against him at Mania 25 but that idea was trashed because it got leaked on the internet. Christian denied another push by Vince's ego( or his business acumen but let's skip that for IWC's sake), how can the IWC not love Christian after that?
I like Christian a lot and that is mostly because I am an Attitude Era guy and Christian was a part of my childhood. Also he was very funny at that time which translated to very good for me at that time. If you notice in all the E&C skits he is the guy who is doing all the crazy stuff, playing the kazzoo or playing sick before Mick Foley and so on. I felt because of this, that Christian deserved his push more than Edge and that feeling has stayed with me ever since. Also this was around the time I was turning smark and so I cannot say that the internet did not shape up how I felt.
But I would say this now. Christian did not deserve a push before 2011. This is not only because he has become better over the years as a worker but because of the roster as well. As I have said before, at most other times in his career, the roster was loaded with guys who were better than him. But that is not the case today and that is why he is getting pushed.