Re: Captain America Easter Eggs Thread *Spoilers*
I'm pretty sure it was because Howard states how it's so rare, the shield is the only amount of it they have. In IM2 Howard states, through video, that he's hidden a special energy source for Tony to figure out how to make because he "was limited by the technology of his day".
The Avengers..The New Avengers
The Guardians of the Galaxy..The Defenders
MARVEL'S THE INFINITY WAR