Specifically, testing on the Nexus 7 has shown a bit of input lag, meaning that player movement is registered slightly after the screen is pressed. Not a huge problem for most games, but a very big problem for Super Hexagon. As Cavanagh puts it, "even a slight control responsiveness issue like this basically kills Super Hexagon – every single touch overshoots." It's possible to play earlier levels, but the higher levels are "unplayable," which makes the game "unreleaseable" on Nexus 7.
The next step for the duo is to test other Android devices for similar issues. Once that's done, the game should be released "very soon for any Android device that can handle it." Sadly, it looks like that won't include the Nexus 7.