I actually only had time to skim it, but there's a cap to nuclear power. At some point rocketry design renders warhead payloads obsolete; it's possible to build a rocket with enough potential energy (which would be converted into kinetic energy upon impact) that it is more powerful than the warhead it carries; this is true for conventional and nuclear warheads. But on a more practical level, you probably just didn't need them. The ones we have are functional enough and can basically end the world as we know it, so why bother upgrading if what we have is sufficient?
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.
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.
I cannot honestly believe that we have not designed new nuclear missile designs since the 70s versions!