Real game changer were ICBMs deployed on both sides in late 50's - early 60's. For the first time in history both conflicting sides were guaranteed to be eliminated nearly totally, no matter who shot first.
After that there was only a slow evolution of such weapons. Increase of accuracy, penetration capabilities (for underground targets), stealth, etc. etc.
All those new toys (including this unmanned sub, highly manoeuverable BM heads and hypersonic missiles) were designed decades ago (and ideas formulated half a century back in time). While there was a strategic balance and several treaties signed and obeyed, there was no need to build them. First, it was costly at that technology level, and, second, raising the political and military tension was counter-productive.
But then the US attempted to change the game back by withdrawing from anti-ballistic missile treaty (in early 2000's) and then deploying dozens of Aegis/SM-3 cruisers/destroyers (and recently "Aegis Ashore"). This could not be left unanswered. Also, the economic and information war against Russia (started recently) already raised tension so high that switching to the next generation of weapons would not add much to the existing hysteria. N.B.: all these nextgen things do not break any treaty.