The second reason is Microsoft, unlike most of its competitors, doesn't make its own devices. RIM, Palm, Apple, Nokia and even Google have their own devices whereas Microsoft relies solely on partners to make phones for them.
If Microsoft wanted to get into the phone making business, they wouldn't have to buy anything though. Google contracts out their branded Nexus phone to HTC. Microsoft doesn't actually make the Zune or XBox 360 either. If they really wanted to make their own hardware, buying HTC makes the most sense as it would be cheaper than buying either Nokia or RIM. In addition, you don't have yet another platform to deal with. Remember, Microsoft purchased Danger which makes the Sidekick. We've not seen much come from that, other than some bad publicity a few months ago when the Danger servers ate user's data.
So the real question is, does Microsoft need to buy another platform to either supplement or replace Windows Phone 7? If so, Nokia isn't the answer. That is an uninteresting platform that is no better position to compete with the iPhone than any of Microsoft's current offerings. RIM is interesting and still the darling to the enterprise customer. However, if the enterprise really interested Microsoft, WinPho 7 wouldn't have targeted the consumer so forcefully. Microsoft is likely keeping WinMo 6.5.x around for a while longer, possibly dubbing it Windows Phone Classic. I suspect that Classic will appeal to the enterprise more than 7 will if for no other reason than there are a lot of different hardware choices whereas 7 basically has the iPhone style form factor.
It makes no sense to me that Microsoft would be interested in an acquisition at this time. The right time would have been in 2008 or 2009 when WinMo was on the decline. They have now put so much effort into WinPho 7 and the release is just a few months away. We'll know in 12-18 months after launch if Microsoft needs to acquire to stay alive or not. For now, I suspect they will hold off.