Let's get one thing clear with the exception of the sweeping hook shot KG has every post move in the book (the basic hook, up and under, fadeaway(his favorite), dropstep, etc) to go with an excellent finishing touch.
Sure he's lost a lot of quickness and explosiveness, but he has the moves if the match up allows.
I believe, KG has become more of a jump shooter in recent years to preserve his legs. To play in the post you have to establish and maintain position(extremely draining), back down your opponent, and then free up the shot with fakes. Add his slight build and height(center of gravity) to the equation and playing in the post is a tiring chore. When KG was in his 20's he could probably battle with the giants all day but now he has to pace himself to stay effective through games (5-5-5 plan). The jump shooting helps save his legs and it doesn't hurt that he's really good at it.
Last year in the second half of the season he made a concerted effort to play more down low and it paid off. A more balanced offense, higher percentage shots and more free throws. Unfortunately for us he eventually ran out of gas and you could see the difference late in games.
Of course we all wish he'd play in the paint more, but I think we all understand why he doesn't.