Perhaps this year the competition was weak ?
"Wherever you sit on the alignment chart, from Lawful Good to Chaotic Evil, developer Larian Studios has written a story to support all your choices. " (from the award text)
Is that true? Not from what I've seen here with complainants about the evil playthrough. I have also seen many discussions around choice actually being superficial - the most important one relating to the end...you know of what I speak. Having played this twice, I agree with that last sentiment...so yeah, the main story is not that convincing.
I thought the main story was average for a AAA story driven RPG (plot issues, pacing, bugs, over the top...), but perhaps the origin character stories the real reason? Given the huge fanbase around the major origin characters and the controversy, like Halsin..always good for business - I think that is probably what they focused on. The game was a huge commercial success (*very* much hyped/feted in mainstream media) and has a large story component (unlike a combat simulator for example) - so it was in the running. And the people who run these 'awards' need to agree with the masses (and large studio, more specifically) to remain 'relevant'.