Well, for one, it's tough on the playerbase to retrofit the position of abilities. It's at best a low-gain kind of change for the stress you inflict on existing players of a champion who have established expectations, and that's really not worth doing. Players are temperamental when it comes to the slightest changes in feeling of playing a champion.
Alistar being such an old champion it's possible there was simply less consideration of assigning the order of his abilities at the time of his creation, but I would also make the argument that he has such a strong core combo with his Pulverize and Headbutt that the QW position feels more comfortable. You could say that together the two abilities make Alistar's primary ability rather than most champions who lead off of their Q as some kind of micromanage intensive ability. Using them in combination requires a lot more concentration, target selection, and purpose.
Well, that's just my thoughts. I know the devs have mentioned something on this subject before, but I don't remember exactly what it was. I think they have said something to the effect that they balance things like familiarity with similar abilities with impact, frequency, and general feeling.