Robert Zemeckis wasn't the producers' first choice to make Forrest Gump; that was Terry Gilliam. He wasn't even the second choice; that was Barry Sonnenfeld.
They both rejected the offer, which worked out just fine for Zemeckis. He crushed it so hard that he won the Academy Award for Best Director.
To be fair, everything worked out just fine for Gilliam and Sonnenfeld, too. Gilliam's next project was 12 Monkeys, which is considered one of his masterpieces, while Sonnenfeld tackled Get Shorty and Men in Black, both widely acclaimed hits. We doubt these guys have had much heartburn over passing on Forrest Gump.