There's probably a well documented backstory on why Oxide choose stm32xx nucleo. I'm guessing VGA signals were not a top priority for hubris
It's at least partially a matter of timing: Oxide was picking its initial hardware in roughly 2020, and the RP2040 wasn't released until 2021.
A handful of people have done ports, e.g. https://github.com/oxidecomputer/hubris/pull/2210, but I expect to stick with STM32s for the foreseeable future – we've got a lot to do, and they're working well enough!
Probably the best history on the choice of going with ARM comes from here: https://news.ycombinator.com/item?id=28989138
The internal RFD on the SP's design still talks about choosing RISC-V, and I don't think (or am simply bad at using search) that the move ended up being in an RFD.