According to athe delay is due to Bungie's ongoing commitment to sorting out the messy matchmaking issues in Halo: The Master Chief Collection.
"While we’re excited about this all-new Halo experience, our commitment to resolving the current matchmaking issues in Halo: The Master Chief Collection remains the top priority," reads the post. "As a result, we have decided to move the release of Halo: Spartan Strike to early 2015. We can’t wait to show you more of Halo: Spartan Strike early next year."
Set during the events of Halo 2, Spartan Strike features the same top-down shooting and driving as its predecessor, with additional new vehicles and abilities, including the Warthog all-terrain vehicle and the Kestrel VTOL, and 30 new missions to complete.
We'll keep an eye out for any further updates.