Buying a major phone company would have been a pretty risky strategy shift. Google's Motorola acquisition is one of Microsoft's best selling points for getting HTC/Samsung/etc. to take Windows Phone seriously, perhaps more seriously than Android. With Windows 8, Samsung and others will have unique incentive to push Windows Phones over Android - they'll have a full lineup of modern phones, tablets and enterprise-ready PCs. Acquiring Nokia would soured a lot of partners who are just as capable of delivering great hardware without taking on the risks of a hardware company.
If Nokia has blockbuster sales with WP7, or other companies with Win8 tablets, expect those companies to be on board pretty quickly. So far, though, it's not happening. Also, those companies know that Microsofts strategy can (and will if needed) change overnight just like it did with PlaysForSure (replaced by the Microsoft-made Zune).
