Probably because Kimi dominated Spa for 6 years. Kimi won in 2004, 2005, (no race in Spa in 2006), 2007, (would have won in 2008 if he hadn't aquaplaned into the wall at blanchimont), 2009.
Without KERS the Ferrari would probably have been an all-around better car. The sacrafices Ferrari, McLaren etc had to make to fit the KERS system in 2009 were simply not worth it. It messed up the balance and was quite a lot of extra weight. Fisi's big mistake that season was to move to Ferrari when he finally had a car with winning potential. KERS made it even more difficult as he was used to a completely different kind of car. He fulfilled a dream, I guess... but it cost him his career.
Kimi's car was considerably slower then the Force India on this track, overall the Ferrari was terrible to drive in 2009. Kimi also came alot faster throughout the La Rouge corner then Fisichella, obviously the KERS helped a little on the straight, but Kimi did a fantastic job overtaking with a slower car, you rarely see these types of overtakes and it certainly was a spectacular overtake, I don't think anyone else could've pulled it off as Kimi did, with or without KERS.
KERS cars were heavier and did not perform well. The best cars of 2009, Brawn GP and Red Bull did not have KERS. The championship was won with a non-KERS car. KERS doesn't undermine this win at all, the Ferrari was a bad car. But Fisichella's performance was fantastic too.
