So as to release the Marvel shows at a steady pace (every 3-5 months or so). We got Iron Fist in March, getting Defenders in August, and Punisher in November (and Jessica Jones Season 2 in early 2018, probably March, then Daredevil Season 3 next Summer, and presumably Luke Cage S2 and Iron Fist S2 in late 2018 and early 2019).
If they release it earlier, that means a likely hole in schedule for early 2018 in terms of their MCU properties. That, and fact they have a lot of other original content that needs to be rolled out at a certain rate to balance audience interest with production time (hence the year span between most of their shows' seasons).