Mainly because so they can get more time to develop the new engine and have more characters being implemented. I'm worried it would be the next SFV when it came out.
Free Step Dodge
JJdude
Rikuto
Currently there is no release date set in stone, just what they are aiming for.
Force_of_Nature
Doesn't matter, DOA6 will release when ready. An updated version can be released later on too anyway.