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.