Yeah that plotline felt unfinished and circular… [spoilers below the cut]
I might have an answer
Re: why did they stop? 1991, is my guess.
August 1991 was the coup that eventually brought down the Soviet Union. Okay so Howard was killed in December, but also December 26, 1991 was the date the Soviet Union ceased to exist.
It was a chaotic time in Russia and one could argue that Hydra wouldn’t let a little thing like the dissolution of the Soviet system slow them down, but maybe they did. Maybe they kept up their experiments until the funding got pulled. Maybe they put the others on ice and locked the doors because better to keep the imperfect ones locked down then to let them fall into the wrong hands (and they might need them later).
They could take Winter Soldier because he was already conditioned and ‘stable’.
For one reason or another, they just never got back there, maybe some like the guy in America went into hiding, who knows. But a lot of stuff went missing when the Soviet Union fell, and that’s true in the real world too.
(and that was what jumped out at me when I saw the year 1991 anyway)