It’s mostly that people don’t value the pros (the thing helps to mitigate climate change) enough, because they don’t care, don’t truly understand how it will affect them, or are selfish. See every thread on HN about EVs - people won’t accept the most minor compromises (cons) to day to day driving.

I think the question about that kind of geoengineering is really going to be a "who cracks first and just goes for it".

My guess is it'll either be India, suffering under heatwaves incompatible with human life, or China beating the sea back from some of its biggest cities.

No Western country will be able to have the political ability to do anything like that as a first mover, though I imagine they'll eventually quietly be on the bandwagon if it works, if for no other reason than it would allow more carbon emissions, which are going to be highly correlated with economic growth for quite some time to come. Whether or not it's actually a good idea, I guess we'll find out in a few decades.