That and the lighter bullets drop off in speed, and also gain speed, much more readily than heavy for caliber bullets. If you're using a 6" barrel, the difference in speed between a 124+P or +P+ and a 147 or even 147+P is going to be drastically different than the speed difference between the two loads in, say a 3.5" barrel.
And the lightweights are designed for speed, so they have to be tougher in order to hold together at the higher speeds than the heavies, which are much better suited to expansion at slower than intended velocities.
I personally think a lot of it has to do with people continuing to stick by the old, sort of kind of accurate for 1988 1000 FPS to expand or not at all rule of thumb. Doesn't work that way in 2010, and it hasn't worked that way since all those wonderful redesigns of JHP bullets that took place over the course of the 1990s. All the premium 147 bullets are superb expanders now, generally they are at the head of the class since designers have figured out how to use the additional weight and material of the heavy bullets to get a wider possible expansion.
And have you guys looked at a fully expanded 124 grain bullet? They are like a fat disc! A 147 that expands to the same diameter or even a little bit wider will have more of its core left behind the frontal area of the bullet, which I can only imagine would greatly assist the bullet in maintaining its path through the target.