What are the reasons for Banshee replacing Rhythmbox in Ubuntu 11.04?
I don't want your opinion on which is best nor why. I want to know what the reasoning was behind the decision.
dv3500a's answer  has links to the information. Here is an informative video  from Gabriel Burt's talk at GUADEC that might give you some information that gives you an overview of some of the things that are nice in Banshee. Here are the slides .
I was at the session and here are some of the highlights:
Banshee has a bunch of other great features , but these were the ones we mostly discussed at this session. http://askubuntu.com/questions/10911/why-is-banshee-becoming-the-default/10928#10928
This was discussed and decided at UDS-N  on Friday 29th October at 10:00 AM. The blueprint for the default application selection is on Launchpad . There hasn't yet been a video posted of the session on the ubuntu developers channel  but there may be in the future. You can also look at the IRC log  for this session. http://summit.ubuntu.com/uds-n/
Banshee has more features, and more plugins (Amazon MP3 sotre, ebook , videos...) and it seems to be more user friendly, even if, today, it's often too buggy. Some reasons are in this blueprint of maverick: https://blueprints.launchpad.net/ubuntu/+spec/desktop-maverick-desktop-application-selection (it has been used for natty discussions too) see in the whiteboard the ==Music Player== part
" [davidnielsen 2010-10-22] Banshee 1.8.0 no longer depends on HAL, it has excellent integration with Ubuntu specific technologies such as the SoundMenu and Application Indicators. The development pace is nothing short of amazing and the community which has grown around Banshee has proven to be welcoming and swift to fix bugs. Banshee has multiple UIs which suits Ubuntu well for deployment on UNE as well as the desktop. "
The cynic in me says: Banshee is a better clone of iTunes than Rhythmbox and given the recent suggestion that Ubuntu 11.04 will have the global menu bar by default, it all makes for a better clone of OSX.