Seem?
Apple does this across all its product line-ups. It couldn't be more obvious.
The exact same reason why there's no Face ID in iPad Air, no Thunderbolt, no four-speaker model, no high-end Magic Keyboard.
This is a basic premise of Apple products, how is anyone second guessing the intentions behind these glaring differences in the different product tiers?
You also think no Pencil 2 or Pencil Pro support on 10th Gen. iPad "seems" like something Apple does to get you to spend more on an Air or Pro?
There's no guessing. It's as apparent as it can be.
I'd guess that moving from Touch ID to Face ID has significantly higher component costs than moving from 60 Hz to 120 Hz. This intuition is partly based on the fact that almost all Android phones have higher than 60 Hz displays these days, but none have depth sensors.
Four speakers instead of two doubles the cost of speaker components.
Some of the Pencil technology is in the iPad itself. I don't know how much more expensive those components are. I've use a few Wacom products over the years, and I know that even when they have styluses that look identical they don't support the same tablets.
I just think keeping the midrange iPads at 60 Hz is more tilted towards product differentiation than component cost when compared to other decisions, like the ones you've listed.
I use the word "seems" because I don't know Apple's component costs.