Yep.
Lots of reasons why, but the #1 reason that I have seen across the board is that by the time *most* kids get to level 8, they are in middle school. My DD started competing level 2 in Kindergarten and did XCEL Silver, L3, XCEL Gold, L6, L7, and L8 starting in 6th grade. She didn’t get a full season of 8 in due to injury, so she is going to repeat but has also talked about doing XCEL Diamond instead so that she can cheer and do track.
So, anyone who started competing later or had to repeat levels (many gyms in my area require 2 seasons of L4) are right in the middle of growth spurts, hormone changes, and opportunities for school sponsored sports, groups, and activities. Lots opt to not advance due to the commitment required for upper levels. Some gyms don’t have the equipment, setup, or coaching staff to properly train above L8.
Some prefer to stop competing after middle school and do high school gymnastics. My DD1 only has 2 or 3 others on her team, we have no L9 or 10 and that’s solely because most have quit to do school sports and excel in sports such as cross country, track and field, and cheer.