If you really don't understand why Harmonix used XNA to facilitate entry into the MS's Live Marketplace instead of developing some sort of framework that has back-end access to Live Marketplace, Wii Shop, and Playstation Network, you're clearly overestimating the sort of access Microsoft, Sony, and Nintendo grant developers into their proprietary networks.Originally Posted by Dennen
The Live Marketplace has two methods by which content can flow into the Marketplace, as opposed to one for both Playstation Network and Wii Shop. The first method for Live Marketplace is the only method for PSN and Wii Shop, which is the standard method by which all Harmonix-developed DLC is adopted for those platforms, which requires the console maker to put the content through a certification process and has a large barrier to entry for content.
The second method, exclusive to the Live Marketplace, is the XNA system, which has intentionally lowered the barriers to entry in order to allow a wider audience of developers to use the service.
If all the content had to go in the front door and pass through all the hoops of the standard DLC process, RBN wouldn't exist because it simply wouldn't be cost effective. XNA, and the way content can pass through the system without the effort of Harmonix, is what enables RBN to exist at all, and Harmonix has decided that rather than leaving the other platforms completely out in the cold, they'll expend the effort to port those songs, so long as the songs appear likely to sell enough to at least pay for the fixed costs of porting them.