I just assumed that Valve ran out of the APUs AMD made for Steam Deck LCD.
There were news/rumours that it was originally designed for Magic Leap 2 and Valve got the leftovers for cheap: https://www.tomshardware.com/pc-components/cpus/steam-decks-... .
If they're going to spend a premium on ordering a new batch, they might as well order the APU for the OLED model they charge full price right?
In general they probably have run out of parts. Set order quantity of everything going on original LCD model. Some like SSD are semi-fungible. But you get only so many custom pcbs you expect to use. At some point you have perfectly functional or superior model and you decide to kill the old one.
I think this is the correct explanation. The OLED version came with a simplified PCB layout and the chip was manufactured on a smaller process, it seems like your standard mid-life console hardware upgrade, makes sense that they were just getting rid of the old stock of release hardware which has finally run out.