I think the thing is that people didn’t really consider Vettel’s achievement in Japan to be impressive because, on a certain level, it was expected. He was fast all weekend, surprised no-one when he took pole, and lining up on the starting grid was as close as anyone got to him during the race. Add to that the way he was in a very good car and has an impressive track record at Suzuka, and it wasn’t exactly an earth-shattering revelation that he got a Grand Chelem.
On the other hand, you had drivers like Massa and Kobayashi who rose to the occasion and put in some of the best drives of their careers to pick up (to some minds, overdue) podiums. They weren’t expected to feature on the podium at all before the race began, but they pulled it off.
To my mind, the Driver of the Weekend award is for the person who impressed you the most. Winning the race from pole, leading every lap and setting the fastest lap time might certainly be impressive, but it doesn’t automatically guarantee you the award. It’s certainly not how I vote. Especially when you’ve got popular drivers like Kobayashi, clinging onto what would be their maiden podium in front of a home crowd.