system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] Why is there only one Winter Soldier?

The Winter Soldier was created when Zola experimented on Bucky, presumably copying some of Erskine's work. The experiment was a complete success, and unlike Erskine, Zola wasn't killed and alive for decades afterwards.

So why didn't Hydra build a legion of Winter Soldiers? They had decades to perfect the process and put it into production, but just stopped.
gpt
There are two reason that come to mind.

1. We don't know if Bucky was unique or not.  While, Bucky was a complete success that doesn't mean that there weren't many other failures.  Zola obviously saw the potential of his experiments so it is reasonable to assume that he did try to create other Winter Soliders.  It could have been that for whatever reason Bucky was special and the only success.

2. Hydra was attempting to stay under the radar.  Hydra was was having to walk a very fine line between being secretive enough to not be exposed and being active enough to grow in power.  Every Winter Soldier they had would require more support and resources.  Additionally, there would have likely been similarities between the Winter Soldiers, this would have drawn attention to the program and Hydra.  A single skilled assassin could be from anywhere; he could be a government agent, a member of a violent NGO, or he could be freelance.  Being the only visible Winter Soldier makes it much harder for people to see a pattern.  And they did a good job of keeping him secret.  Black Widow only knew rumors about him, and Nick Fury didn't seem to have any particular information about him; though being Nick Fury he may have withheld information.  A single Winter Soldier best fulfills one of Hydras man concerns; not being exposed until they are ready.