Turned out, trying to place it in two series as both a prequel to TOS and a sequel to Battlefield Earth didn't play well to the test audiences.
Possibilities that occur to me, some of which have been mentioned:
1. The show isn't ready or needs reworking somehow.
2. CBS All-Access isn't ready and needs reworking somehow, or, CBS wants to start with more new content that they're still developing *aside* from Discovery.
3. CBS believes that Rogue One or some other property from a competing company is going to be big in a way that would detract from the launch, so they're moving the launch to avoid overlap.
4. Turns out, Discovery IS based on the Battle of Axanar, and now *they* have stolen some of the little bit of *original* IP that Peters & Co had created and have been hit with a cease and desist.