Switching from FPTP to IRV changes the balance of power in any state that is not dominated by one party. The party that stands to gain supports it, the party that stands to lose opposes it. That's it there is no other motivation. If the balance of power were flipped you would see the stances flipped.
You can see the happening right now where the Democrats who initially opposed those state laws meant to undermine the ACA now loudly support them because almost all of them banish government from healthcare extremely broadly where Republicans now want them gone because it inhibits their ability to restrict abortion access.
I guess we have to work state-by-state and county-by-county in swing areas to get IRV/ranked choice voting enacted, then scream very loudly when someone tries to change it back.