Perhaps because they don't want Nvidia to pre-empt such a move before they can get it to market.
Perhaps AMD think they can get it on the market close to a 1070 in performance @ $300 vs $375 when the 1070 is out and set at that price.
Nvidia have already made their move and the card is out in 3 days and from what we have seen by the 1080 launch, 1000's of people will buy one. Some of which might have gone for or waited an extra few weeks for this fabled 490.