Why didn't I earn a hex chest?

Hello -

I'm not sure why I didn't earn a hex chest from this: I played a 1)normal blind pick game in Summoners Rift, 2) I'm playing a hero that I own but have yet to earn a chest with (veigar), 3) although I got a B rating, three of my teammates got "S-," 4) I have 4 chest available to me. After the game i didn't earn a chest, can someone help me understand why?

Thanks, CoogBear