Eh, I can buy that even with
the notes and research maybe only "certain people" can make these things work. Dr. Erskine may have "shared the formula" but it could also be like when your grandmother bakes her famous cookies. There's more going on there than the numbers on a piece of paper and it takes a special touch, nuances and being able to judge the relative humidity to determine how much flour is really needed.
Erskine just had the "knack" for making it work and it was something no one else was able to duplicate even with
the formula and H. Stark's technology.
And H. Stark seemed to purposefully
hide the secret to the new element to revolutionize the arc-reactor specifically as a legacy for his son to take on.
So, I can see how specific people's techniques and such aren't able to be duplicated or recreated but here we've got SHIELD/the US Government and H. Stark with the Tesseract and all of Hydra's weapons and they can't figure out what to do with it over the course of 70 years?
(I guess to be fair, the Hydra factory was
destroyed in the process which may go along the same thing that happened with Erskine and H. Stark and the new element. It was a missing piece to a puzzle and it couldn't be duplicated.)