While the move may be explained by purely business reasons - i.e. losing money in a difficultly competitive market with ever-diminishing margins (partly brought about by Android itself, of course) - these points would have been valid before the acquisition.
The real reason, I feel, can be found three quarters of the way down CEO Larry Page's blog on the subject:
Google will retain the vast majority of Motorola’s patents, which we will continue to use to defend the entire Android ecosystem.
A lot has been made of the reported $9.6bn hit Google is taking on the loss-making division (it bought for $12.5bn and is selling for $2.9). But that probably just indicates the value that Google puts on the patents. They are not part of the sale...
Still, it's a shame. I thought Google was going to use Motorola as an example of Android best-practice for other handset manufacturers, and the excellent Moto G (pictured) was a sign of better things to come.