Tourism accounts for, I believe, 15-20% of Greek GDP. It is one of THE pillars of the Greek economy. Actually, all efforts should be made to expand it because it generates income from abroad.
What really makes matters worse is that the above declines come against a background of increased tourism spending overall. So people are spending more on tourism but they are spending it elsewhere and not in Greece. That is a very dangerous development because it could feed upon itself!
Presumably, the declines have to do with the deteriorated image of Greece. Whatever it is, the year is still young and all efforts should be made to turn this development around.