It's a nod in the direction of social science, rather than economics per se. It's another homage to the New Institutional Economics and also to Law and Economics. It's rewarding larger rather than smaller ideas, practical economics rather than abstract theory. It's a prize somewhat outside of the mainstream. As you probably know by now, Ostrom is a political scientist and she has spent much of her career at Indiana University.
I was delighted to hear of Ostrom winning (which I had not expected) but frankly it makes the omission of Gordon Tullock all the more glaring.
Here are interviews with Elinor Ostrom (recommended). On Elinor Ostrom, here is Peter Boettke and on Williamson and Ostrom here is Lynne Kiesling. Here are varied reactions. Here is an excellent list of long links on Ostrom. Here is Henry on Elinor Ostrom.