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:

• Macarthur could have good reason to change their priorities. Nuclear work may just be super intractable. Maybe we can still make much more progress on other issues.
• Macarthur has funded 88 other organizations in nuclear issues in addition to NTI. EAs are aware of NTI because orgs like Open Phil have supported their bio work previously, but it would be good to look at the other orgs that Macarthur funded too to see who else is out there. With 89 orgs to choose from, it's plausible that NTI is not the best funding opp at the margin. But working out which funding opportunities would be most valuable at the margin is a lot fo work.
• Macarthur represents about 45% of total funding in the space. That's a lot, but I'd expect the remaining 55% to be shifted around a bit and hopefully cover the most marginally-valuable opportunities

To respond to some of your specific points:

• I'm unsure how relevant the "EA has a lot of money right now" point is. There's lots of stuff to fund, and saving can still be good because (1) we may learn a lot more about good stuff to fund in the coming years and decades and (2) the fields we're pretty sure are good to fund are still growing, and it might be worth saving our money so we can grant more to those fields in the future.
• There's a war going on now, but I'm pretty sure there's nothing NTI can do to reduce nuclear risk  right now. The question is whether we think total risk from nukes in the medium-to-long term has increased. Or these issues might become more tractable to work on as they're more salient now. This might make funding the work of NTI and similar orgs more attractive. But it's complicated.
• Not sure I understand the point about "hiding it" - are you asking if there are plans to fund this stuff that funders just aren't discussing yet?

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.

At SoGive we've just recently started looking at anti-nuclear weapon orgs to try to determine whether it makes sense to recommend any of them to donors we work with, precisely because of the MacArthur Foundation's withdrawal.

The MacArthur Foundation has been funding a large number of orgs in the anti-nuclear space. While NTI have a good reputation and are known in the EA community, I don't think it is obvious that it is they who should receive funding rather than other organisations. 

And these orgs aren't necessarily interchangeable. Based on a couple of expert interviews, there doesn't seem to be one widely accepted theory of change as to how to best approach reducing risks, and some approaches appear contradictory. My guess is that some of the big EA funders are also trying to do some research in this area before making any donations?

I know that at least one EA funder has seriously considered funding NTI, and more broadly considered whether to ramp up funding in this area given that, as you say, the biggest previous funder (the MacArthur Foundation) is going to pull out. I don't know about the status of their decision, nor any other details.

[Mainly sharing this to provide one data point on how 'efficient' the current funding ecosystem is.]

I don't know why big EA funders did not support NTI's work in nuclear security after the MacArthur Foundation decided to end its funding.

As Joan Rohlfing, NTI's President and COO, explains here, the Foundation's new President (John Palfrey) and Board chose to terminate their support apparently after concluding that other areas, specifically climate change and social justice, were more important.

Note that the Foundation is not pulling its funding immediately. As this Politico article notes, "It is engaged in a three-year, $30 million 'capstone' effort to tackle nuclear challenges before it winds down the support in two years." I wonder if, in light of recent developments, they could be persuaded to reverse the decision? (As a measure of what the Ukrainian crisis has made possible, consider that Germany is now open to the possibility of extending the lifespan of its three remaining nuclear power plants, which were set to shut down permanently at the end of 2022.)

EDIT (18 March 2022): This Future Perfect article by Dylan Matthews has additional details.

Turns out we are. 😂

Also i’m gonna edit to change the “hiding it”framing. Decisions were being made. That was fine. It was also fine of me to ask.