Well the political situation changed. Military policy is dictated by political policy; the ideas of easing tensions and disarmament are now what's important in nuclear terms, from a political POV. And it's basically been that way since the 70s, so no investment is going into that kind of research.
There was 2.3 Trillion missing from the pentagons budget last year. I don't believe most of that the government/media reports.
The government doesn't report it because obviously, and the media doesn't because it's much better for ratings if you have sound bites and intense (not-at-all setup) debates between people who disagree, and so on. A milling "X amount of dollars" and then speculating and where it could be, and asking for a probe or some shit - only for the probe to come out and say "Blah blah blah, nothing to see here" - doesn't keep viewers gripped. That's where we are now, Yellow Journalism.
EDIT: Fixed a sentence.
If you are making such weapons you would think they would constantly be made as deadly as possible. Well I would. Faster so you can strike first.