Why does the advancement rate seem to slow down as a SM progresses higher in rank?

Just for clarification, a lot of the topics mentioned in this thread are intertwined. Yes, the military does have less of a need for higher-ranking people (pyramidal rank structure). That creates an environment where promotion timelines slow as you compete for schools and developmental assignments, thereby ensuring competitiveness when people are jockeying for fewer and fewer senior-level positions. You also have mandatory requirements for time-in-grade. These requirements help ensure that people have an adequate foundation and the professional development necessary for sustained success at the higher ranks (where authority, responsibility and accountability are greatly magnified).

Man sir, I started out as an E1 and got my degrees later on. Well played.

CPT (Join to see) it's amazing, I got fired from insurance company job unfairly and I reinvented myself by going into the National Guard as enlisted member of military police back when it was 95 Bravo at age 31. I already had a bachelor's degree so by age 33 years 9 months I was commissioned. Here I am 22 years later and I just made it to LTC. Never in my wildest dreams! Sometimes it is true that behind something bad there's something good that results from it. I never expected to be in the military when I was a teenager or a young adult.

I have met lots of folks--mainly medical services corps but also other medcomm jobs-- and the phrase I heard over and over again was "everyone at one time or another was just another knuckle head in the barracks". And to that I say you gotta start somewhere. As I right these words we are in the full grip of a pandemic. And the armed forces needs nurses. Look into "Green to Gold" program. The army pays soldiers to attend the final two years of a BSN degree program and once you pass the licensing board you are 2LT. If you go on to become a Nurse Practitioner (similar to a PA) or CRNA (nurse anesthetist) you are more or less guaranteed to retire as LTC or higher.

^^This. I'm not sure how a platoon is structured in a support unit, but I can tell you that an Infantry Platoon has slots for 18+- E4 & below, 6-E5, 3-E6, 1-E7...multiply that x 3 plus HQ Platoon, and you have a company, which only has 1-E8. Simple math is part of it...there just aren't as many leaders as the levels move upward.
The other part is experience. It takes a lot more experience to be a team leader than a saw gunner, and a lot more to be a squad leader than a team leader, and so on up the line.