If Delph left last week, he'd have been booed by villa fans twice next season. By leaving now, he's surely going to be hated by every set of fans in the country?! (Except Blues I guess...)
The only thing I can think is that Man City have offered a huge increase on their original offer. So at first he was like "umm, y'know what, I don't really want to leave so I'll stick it out". But now - coupled with Benteke going and no takeover - the new offer is just silly money that is too much to turn down.