A large motivation for this move is likely to ensure that attempts by some incumbent ISAs to lobby the US government to curb the uptake of RISC-V are stymied.

There appears to be an undercurrent of this sort underway where the soaring popularity of RISC-V in markets such as China is politically ripe for some incumbent ISAs to turn US government opinion against RISC-V, from a general uptake PoV or from the PoV of introducing laborious procedural delays in the uptake.

Turning the ISA into an ISO standard helps curb such attempts.

Ethernet, although not directly relevant, is a similar example. You can't lobby the US government to outright ban or generally slow the adoption of Ethernet because it's so much of a universal phenomenon by virtue of it being a standard.