I suspect regulators and incumbents have been pushing the "inertia" argument. Regulators don't want to admit culpability. Incumbents don't want new sources competing.
The reality is that it was a regulatory failure of the designers of the wholesale electricity market rules (and rules for adding new generation sources).
> It’s possible to make up for the [lack of inertia for wind and solar generators] with cutting-edge technology such as advanced inverters or synthetic inertia. But many solar and wind farms haven’t undergone these expensive upgrades.