Before we go to why or why not, I'll like to add the fact that I and the rest of the world is devastated. Now read...
According to the show's writers and producers David Benioff and D. B. Weiss , they said they had to push the release date to a much later date when the weather was much grayer as opposed to summer. If you remember Sansa and Jon's last episode in the previous season, "Winter is Finally here" and therefore it can't be shot with the sun all up. The change of the season means that the production needs to be pushed back a little bit so that the series can be shot with natural grayer atmosphere.
Note that till date, the premier of season 7 is yet to be known, meaning that Game Of Thrones Season 7 will air much later than fans expected (noting that the series usually premiers April of each year). According to Benioff, he said " We're starting a bit later because at the end of this season, Winter is Here --- and that means that sunny weather doesn't really serve our purposes anymore". I must commend and second that, Winter is Here, the last thing I want to see in the following season is a budding plant. Even though we have to wait a little longer for the seventh season of GOT, remembering that there are two seasons left; both having short episodes of seven and six respectively, you still have to commend the producers for their hardwork and commmitment to delivering the best of entertainment. So therefore if we have to wait a month more or less (fingers crossed) to get the best props, aesthetics .... then a man must wait. Wait we shall.
In addition, you most definitely know we will see more of the White Walkers because of the dedication to "grayer areas" even in sunny scenes. That I am definitely looking forward to and you should to.