I think the new ceo has been appointed through the course that Microsoft has been taken the last years.

People internally has been pushing Microsoft the open source way (some evangelists of MS and .Net). They have gotten greater influence into the firm, because of success and a allround positive influence (and Microsoft didn't had any appeal to a great group of developers because of closed source and patents).

The similarity between the new CEO and those people where greater then with Balmer. So it's a logical step into it's evolution.

It doesn't mean that Balmer hold them back at all, they could thrive under Balmer, but perhaps he hasn't had any experience with this business direction or couldn't grasp the concept of it. And as a result, someone else was more fit for the job.