It wasn't ready yet. A significant amount of development happened after the point where that's supposed to happen for features to be integrated in 9.5.

Now that 9.5 has its own branch I hope we'll get it in early in the 9.6 development cycle; so we have time to iron out all the potential kinks.

I don't think there ever was a clear "this will be in 9.5" statement from anyone?