Mainly because I think the pressure coming from the rest of the board (by all accounts they all want him gone) and his son (who's in London on a "fact finding mission") would just be too much for him to ignore anymore. Plus at this point even the most optimistic fans have gone - I think there's barely a dozen Arsenal fans among the entire global fanbase that want him to stay.
I think it's just the respect they have for Wenger that's preventing them pulling the trigger until the summer. Arseblog was making a good point about this but it seems unlikely that Ozil would have resigned unless he was given assurances about the future direction of the club, including change in the summer.