The bigger question is why you would think they would already announce TES6 when they just did Fallout 4 in 2015.
Anyways Todd Howard and Pete Hines have said multiple times it is many years away.
2019 fits the rough pattern: Fallout 3 (2008) Skyrim (2011) Fallout 4 (2015) TES6 (2019)
They dropped big hints that TES6 will be a pretty deal. I expect it to have the settlement building from Fallout 4 and be fully VR compatable (Todd Howard showed off a Fallout 4 VR conversion last year. This was the proof of concept).
It's also possible that this game isn't TES6 and is a different, maybe new IP (they've dropped hints at something big that ISN'T TES/Fallout, and I am asuming they weren't talking about Quake and Prey). Meaning TES6 will come even later than 2019.