Cap was a 98lb weakling before taking the serum. Doctor Erskine, head of the super soldier experiment was impressed by his tenacity and willingness, however if it was unmatched by his feeble body. He aimed to not only create super soldiers from regular schmucks, but also to turn the flat footed sickly rejects into soldiers who could defend their country as well. Thus the serum went to Rogers.
Behind the good doctor's back, it also went to another man, McIntyre. He was a healthy and strong man, but had been repremanded for misconduct. A general gave it to him, thinking that administering it to weak people like Rogers was a mistake. However, I think Erskine had to give the recipient a special innoculation, which the general failed to administer, eventually killing Protocide. He was put on ice however, later found by A.I.M. and revived. Hasn't been seen since some of the later Cap v3 issues.
Tests for the super soldier serum were originally given to enlisted African America men. One of these men, Isaiah Bradley went on to become a black Captain America before Rogers' time. The serum was unstable though, causing brain damage. However, it was passed onto his son Josiah X, who has super soldier abilities.