Why does it seem that all SMP cadets are Cadet 2LTs?

Because it's all made up rank anyway. I only ever wore the CDT 2LT rank as an SMP because anything other than that gets confusing as your ROTC unit rank and position has no impact on your guard / reserve rank and position. The reserve component only has one rank for cadets, the rank of "Cadet"

Because the vast majority of cadets who are seniors will be Cadet 2LTs.

However, only seniors have "cadet officer" rank, so any SMPs cadets that were Sophomores or Juniors will have "cadet enlisted" rank.

If you're referring to the cadet rank they hold with their ROTC units, it's irrelevant. Each BN has their own policy on how they want to structure their ROTC unit, but from what I have seen, the general guideline was to have each MS year receive progressing responsibility, so you get cadet rank corresponding with your duty position. It had no bearing on your achievement or capability. All of it was actually assigned with a focus on development.

For example, regarding the MS3s, does a cadet struggle with coming out of their shell and verbally leading others, make them a 1SG. Did a cadet miss out last semester because of sports but has shown great leadership potential, give them a difficult position this semester and make them prove themselves, like battalion commander as LTC. Did a cadet do just about everything right last year and just needs general development, throw them in a staff position like S4,S6,S1 and make them a CPT. We're all the positions filled and the cade is now just in the background, make them an LT and put them in the staff.