It's just political maneuver. The conservative London mayor candidate is against it
A cynic might point out it helps them out of a political hole. Conservative MP Zac Goldsmith said he would resign and force a by-election if they picked Heathrow. But he'll be tied up with the London mayoral election, which is to be held on 5 May 2016.
and they don't want to announce that before the election.
One could infer that the decision will be Yes, but they will wait until after the election pans out.