(I work at Oxide, though I wasn't around for the initial chip selection process)
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!
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.
