Why would Microsoft care about Nokia, specifically, running Android?
Shouldn't they have wanted to buy Motorola since they were the early leader in the Android market?
It's about market penetration. Nokia had a way bigger brand outside of the United States. It's basically a household name.
Motorola? Not that much. The Droid series barely made it out of the US and Europe.
There are bigger markets (Asia, China, India, Africa, Middle East) where Nokia is pretty popular and Moto is unheard-of.
Nokia + Android in these countries is definitely a recipe for disaster (for M$, in this case).