If they had one (and all reports indicate they do) then it would have been suicide to release it before now.
The last JB was for 8.1.2. Had they released with 8.1.3, Apple would have patched with 8.2 and above and they would have had to start all over again. A blown jailbreak that only works for 8.1.3?
If Apple announces iOS 9 tomorrow then we wait a little longer for the iOS 8.4 release and then the jailbreak gets released. That means everyone from 8.1.3 to 8.4 can jailbreak.
Apple may patch in iOS 9 but in the meantime way more users are able to update and jailbreak and iOS 8.4 stays signed until the release of iOS 9.
It's frustrating to wait, but this is the game we all play for the benefit of having a jailbreak that gets the maximum amount of time it can have on the scene while letting the most people possible utilize it.