Because they want to get their music streaming service off the ground ASAP and redesigning the music app for that makes perfect sense. They would not go through adding features to the music app in every 8.4 beta if they didn't intend on releasing it soon. They would have saved it for the iOS 9 betas. Plus it also makes no sense for Apple to put a new music app in the 8.4 beta when its intended for an iOS 9 release therefore removing any wow factor. Are you forgetting 8.4 is in PUBLIC beta? Trust me it will be part of 8.4
It might be a Public Beta, doesn't mean its gonna get released. The streaming service may be released as a separate app, none of us have any idea what Apple's going to do, do we? If you have used the 8.4 beta you know unless Apple has gone a bug squashing mission over these last few weeks, it's not ready for release. There are still broken things in the music app, although they are getting better. Up Next still frequently crashes on me and unless they yank some features out before 8.4 drops, there are going to be a lot of unhappy users.
Also, I am not saying the updated Music.app won't be released in the final version of 8.4, I'm saying 8.4 might never be released period.
Apple have already said they'll be releasing an update soon, which means for iOS 8 as September is still months away.
The messaging bug would probably have been fixed by now if they wanted it to be, so I think they've added the fix to 8.4 which will be released shortly.
Apple could very well release 8.3.1 to fix the text message bug, we still have 4 days until WWDC. Heck, maybe 8.3.1 will be released at WWDC, we have no idea. I'm just guessing here.