I would posit a couple of reasons, one common one and another that affects a small program like the one I run:

1. Many high level 2028s will be committed by mid October. Might as well play two years at 18u if you can. It definitely is a better level of play.

2. With the age rule change, only Seniors have to play 18u. My typical graduating class only has 7 to 9 players in it. I can't field a team with just Seniors. Last year, our top 18u pitcher and catcher could have played 16u last summer, but they got recruited early and we needed them to play 18. In addition it freed up innings for players at 16u still in the recruiting process. Both teams had Top 10 finishes at nationals so what we did was (at least arguably) in a lot of players individual best interest. If you have a larger program this may not be a problem, but it will be a recurring issue for me.

I agree, if you are in the recruiting process you will see more coaches at 16u. If you are already recruited you will be tested more at 18u.