Back in the days of the flat track crawlers, you needed an ag model crawler if you were going to use it for farming. A different model was required for dozer work. Unless you paid attention, it was hard to see the difference until you removed the dozer and tried to farm with a dozer tractor. The construction machine was way back heavy because it didn't have the blade to counter balance it and the down force from the drawbar load made it even worse. Likewise, if you happened to find an ag CAT and think you could fit it with a blade, you'd be disappointed also because then you'd have a machine that was nose heavy. This balance was determined by how far ahead or back the track was. As you might expect, the machine had to be special built to change the balance for different applications. The high drive machines were easier to adapt to differing balance requirements. While still no trivial task, all you really need is a different track frame and mounting point and you don't need to change the chassis like was required on the flat track machines.
As has been mentioned, the planetary finals on the high drives are much more robust than the old bull gear and pinions that were found on the flat track machines. While I guess you could mount a planetary down at ground level, it would take more gears to accomplish it than you need when just mounting the drive up high.