At the Mobile World Congress show in Barcelona, Nokia CEO Stephen Elop explained the rational for Nokia’s partnership with Microsoft. He basically said that Nokia did not chose Android because doing so would have concentrated too much market power with Google. Nokia is trying to make the smartphone market a three horse raise between themselves, Google, and Apple.
From what I read, it sounds like mobile carriers wanted Nokia to chose Microsoft over Google because they don’t want the smartphone market to come down to just two companies. From the carrier’s point of view, three companies with relatively equal market share provides them leverage over those companies.
If one smartphone platform became dominant then the company behind that platform would have leverage over the carriers. We saw how that could affect the carriers when Apple first launched the iPhone with AT&T. In order for AT&T to get and keep their exclusive deal to sell the iPhone, they had to concede a lot control over how the iPhone was sold and updated.
What I really think Nokia is hoping will happen is that the carriers will back the Nokia Windows Phones because it’s in the carrier’s interests for them to succeed. The strategy might also improve Nokia’s relationship with U.S. carriers, who have not really worked well with Nokia in the past, because again, Nokia wanted more control than the carriers wanted to give them.