Now if only Criterion would upgrade The Irishman to 4K.

I wonder why they have not done it so far?

Because that's Netflix, not Apple. They probably want 4K exclusivity on the Netflix service or something along those lines.

Guillermo del Toro's Pinocchio is from Netflix and that got a Criterion 4K release.

Different releases from (slightly) different times during Criterion's transition period to 4K, and also different deals with the streamers, directors, etc.
Criterion didn't jump into 4K until late 2021. Going back and doing 4K editions of titles released between 2016 and 2021 may require re-licensing, which is why some titles ended up at other distributors for some time, or haven't been prioritized for 4K.

Nothing top-tier or flagship worthy. Scorsese is always top-tier and worthy. And Scorsese probably relinquished his rights somehow for a physical release. GDT doesn't make 200 million films, so it was easier for (probably) to negotiate a future CC title.

The Irishman was released on Blu-ray about a year before Criterion started doing UHD. What's the most recent Criterion Blu-ray that has received a subsequent 4k upgrade?

Blue Velvet, maybe? I'm going by any spine number before Uncut Jems, Citizen Kane, and Menace II Society, since those were their first 4K releases.
The Blu-ray was released May 28, 2019, and the 4K was released June 25, 2024.
But working backwards from there, there are a bunch of pre-spine 1101 (Uncut Gems) that have since had 4K updates.