I think the reason for planned XMR.to integration first is pretty obvious. That is a community project and Monero focused. Shapeshift, though we are all grateful that it supports Monero is far more multi coin focused. There's nothing wrong with that and I would be very surprised if I shape-shift plugin doesn't come along pretty quick. I also assume that the plugin architecture will be open source. So whoever wants to can make a shapeshift plugin.