In the official tie-in comics, Erskine tells Phillips that the formula for the serum exists only in his head. He never wrote it down so that others wouldn't misuse it. That's why nobody's ever managed to successfully duplicate it after Erskine's death. I agree that it seems odd how the SSR/SHIELD never cracked the Tesseract despite having it for seventy years. Maybe it would have been better if the Tesseract had been found more recently.