Why did FTD choose Hermes as the logo?

Iris was the original messenger god in the greek pantheon. She was the rainbow that traveled from the earth to the heavens bringing her news. Over time she was replaced by Hermes, god of writing/magic/journey to the afterlife. It's the journey to the afterlife part that's the nearest to Iris.

FTD is in the business of delivering colorful "messages" in the form of flowers ("Get Well" "Congratulations" "Sorry for Your Loss"). Flowers are impractical short-lived decorations, meaningless if not for the cultural associations of "a dozen long-stemmed roses" (love) or an assortment of calla lilies (bereavement). These are messages in material symbolic form.

You might argue that Iris would be a better symbol for FTD, what with the rainbow association and all (nature, colors), but no one knows who she is just by looking. Best they could do would be a "rainbow lady," and that wouldn't express the quick/convenient end of the business FTD wants to communicate. Hermes is very recognizable because of his hat/staff and other traditional iconography, but he also looks like a nearly-military man on a mission: a good impression to give people of your workforce when you're advertising getting the goods there quick and right.