Android provides a platform for hardware manufacturers who are not in the software business.
The point why Google bought Android (the company) was because it showed potential to become a widespread phone OS and thus a way for Google (which is a advertising and data mining company, their products are mere tools to reach their goals) for getting a strong hold of the evolving market of mobile advertising.
Putting Android on a phone only helps Google in the end, and is only an option if you are in need of a ready-made competitive OS for selling phone hardware.
Nokia's intention is to become a more services-oriented company. Android wouldn't fit into that picture. Not at all.