I don't think lack of participation from other browsers has been a big component. Firefox actually gave some excellent feedback on the FLoC API. [1] Instead that it's just been really hard to get these new privacy preserving APIs working well. Turtledove/Fledge, for remarketing, in particular, is extremely complicated and not done. Making something fully privacy preserving that recovers no revenue isn't especially difficult, and making something that recovers revenue but has fatal privacy issues is not that bad, but solving both of these at once, while building a system that has good performance on real devices, is somewhere between really hard and not possible.

(I used to work on this, on the Google Ads side)

[1] https://mozilla.github.io/ppa-docs/floc_report.pdf