Choosing a 32-bit microcontroller
I think the best answer to this question, which gets asked a lot, is to look at a good survey. Embedded has a very good survey and it includes many factors.

Look at it and you can see many of the factors motivating people:

UBM Embedded Market Study 2012

Some of the factors you mentioned, and many others are covered and reveal  a wide variety of factors play into the selection. Familiarity with the parts dominate, but the strength of the ecosystem is very important.