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".
Iron Man..Hulk..Thor..Captain America
Black Widow..Hawkeye..Quicksilver..Scarlet Witch..Vision
MARVEL'S THE INFINITY GAUNTLET