re:
Consider when Apple released the iPod. They were originally only compatible with OSX, and they were popular, but after making them available for Windows PC, sales exploded. They realized how crucial it was to make this product available to the dominant platform at the time, Windows PCs. So they did, and were wildly successful.
While MSN isn't a product like the iPod, they got to realize that if they want the product to grow, it has to be available on the dominant platforms, in this case, it is Android and iOS.
Having Microsoft products available on other platforms doesn't make them perform worse on Windows.