As opposed to the closed source Windows Mobile kernel, so it's harder for mobile manufacturers to customize the interface, not to mention the mobile service providers.
I see this as an act of desperation, since custom Linux builds can do the same or better than Windows Mobile, are cheaper and more flexible.
The notion that mobile phone operators complain is laughable, they sell lots of different kinds of phones. If they really wanted one operating system, they would choose one and only sell that one in their webshops. They certainly have the power to force something like that. Palm, Blackberry, Symbian, lots of custom flash builds, Windows Mobile, etc. have been existing along side for years. It shows that multiple operating system support by mobile phone operators obviously isn't that much of a draining resource.