In what seems to be a fairly arbitrary move, Google has trimmed the fat from the Play Store's Nexus 7 cover selection. The lineup, which formerly included pink, light blue, and gray, has been cut down to just the light blue color in the US, with the pink cover variant appearing outside the US. The gray cover has been axed completely.
It should be noted that this only affects the Play Store's own lineup of covers. If you're a Nexus 7 owner still looking to wrap your tablet in an official ASUS cover, the manufacturer is still advertising the full lineup of colors on its website, and a handful of colors (light gray, dark gray, pink, and light blue) are available from Newegg here.