Just to clarify my tweet, I was saying NTI seems like a reasonable place to donate if you want to support nuclear security - as a proof of concept there are useful things to be done. I'm not claiming (i) nuclear security is the best cause area to support at the margin (in general I'd rank AI safety and biosecurity higher) or (ii) NTI is the best place to donate to help with nuclear security at the margin. Overall it seems plausible to me that donations there are 1-10% as effective as the EA Long Term Fund, and so below the bar for core longtermist donors (though still more effective than like ~99% of charities).
I haven't looked into specific nuclear orgs so am pretty uncertain about this, but suspect there are probably good funding opportunities in this space.
To speculate on why no funders have stepped into the breach, though:
To respond to some of your specific points:
Again, I'm on the whole sympathetic to your view. I'm not sure how many EAs should be thinking about and funding nuclear/conflict issues, but the answer, IMO, is not 0. But I do also think there are good reasons not to rush into the space, and it's not obviously wrong that no one has stepped up to fund NTI.