A few months ago, Dolph Ziggler turned heel after being frustrated by a string of losses, and then... continued to lose. Despite feuding with the likes of AJ Styles and Shinsuke Nakamura, The Showoff never really received any kind of push (so what's new?) and hasn't been seen on WWE TV in quite a while.
There was speculation that Ziggler might be injured, but apparently that's not the case.
We're hearing that creative simply doesn't have any ideas for Dolph right now, and feel it's pointless to feature him on TV until that changes. There is also speculation that there are plans to repackage his character.
Ziggler is one of the most talented guys on the roster, but the WWE higher-ups (*cough* Vince McMahon) just never seem interested in pushing him to the next level. Hopefully things will be different upon his return - but if he is repackaged, let's also hope that his new persona isn't something woefully stupid that'll get him buried.