Most people learn to juggle with their feet while standing first, so lowers tricks are the most natural tricks to learn first. Also, most freestyler's first trick was atw or cross over. It only makes sense to most people to improve on this area the most. It's also quite easy to come up with new ways to push the limits. You can always find one more revolution around the ball. Landing the trick might be more difficult than coming up with it.
Sits and uppers have a bit of a learning curve. It's harder to juggle with your upper body or with your feet while sitting than standing with your feet. For these reasons, I think people are less likely to put huge focus on uppers or sits over lowers. Also, pushing the limits in sitting and uppers requires a bit more creativity with the tricks involved. It's not like lowers where you are doing mainly atw variations. You must come up with different types of tricks. stalls, juggles, spinning, rolls, etc.
Grounds seem to be the style that is growing the least of any style. It's quite difficult to make grounds look good, so most people don't do much. Pushing the limits in grounds is very different from other areas. Also, not near as many people are interested in grounds. Not taking away from grounds, but most people don't appreciate them for more than a few seconds.
