I have a pet conspiracy theory for why there has been so little progress in physics for so long. The invention of the nuclear bomb scared a lot of people, it made them scared of physics. What else might physicists turn up that could change the world in dramatic ways? Anti-gravity? Ray guns? Other dimensions? Travel to other worlds? All bad for business, no one is going to buy your airplanes or air craft carriers if t…
If we're talking about 01995, it's conceivable that, say, the US and CERN could coordinate to suppress research into hafnium bombs, AVLIS, antigravity, or whatever. If we're talking about research much prior to that point, though, you'd have to include the Russians in the conspiracy. Probably not just any Russians, either; probably Khrushchev, Brezhnev, Andropov, Tsar-Bomba-era Sakharov, and his successors. And, on the other side, people like J. Edgar Hoover, JFK, McNamara, Kissinger, Johnny von Neumann, and Teller.
I don't want to say it's literally impossible for Brezhnev or his underlings to have made a secret agreement with Kissinger and Teller to suppress the development of theoretical physics in order to keep the world predictable. But I do think it's pretty implausible, and there would have been an enormous incentive to cheat on any such secret agreement.
In the 01990s, though, it could have become plausible. But, remember that that's also when Pakistan became a nuclear weapons state, shortly followed by North Korea in 02006. And the People's Republic of China has had nuclear weapons since 01964, so they evidently had significant physics capabilities that they were willing to use for warfare (which was a huge priority; Mao reorganized the country's economy to resist an anticipated US invasion), and they dominated the TOP500 supercomputer list until this year, when they withdrew from it in apparent protest against the efforts of the USA to reverse their technological progress with a worldwide system of export controls.
So I think there's maybe a ten-year window when this could have happened somewhat, about 01992 to 02002. Both before and after that, there are too many countries with strong physics communities that are too bitterly opposed to make such cooperation plausible.