Although XBLA has a larger audience, the combination of the royalty rate cute, the difficulty in getting submissions approved, the sheer volume of competition on the platform, and consumer expectations holding down price ($10 max) make other platforms more attractive right now, unless of course you are a publisher whom MS is giving the old rate, or you are a developer who works with one of those publishers.
One of the great things about XBLA was that you could do it without working for publishers, who are essentially the pimps of the game industry, making money off other people's work without providing much of value in return. Developers who did not need the one worthwhile thing that publishers provide - financial security - could bypass those parasites and succeed on their own. Now a self-funding developer gets a royalty rate that's not only not great like it used to be, it's worse than the competition by a long shot.