well, there is an element of good and bad luck in every sport. it just so happens that (if i'm correct) there is more luck involved in keirin. or more opportunity for good or bad luck determining your outcome.
and i don't think luck and strategy are polar opposites. what about poker or blackjack? you have to deal with the cards you're dealt, but there are obviously good poker strategies and bad ones.
The aspect of luck, and the possibility that anything can happen is the reason why keirin is such a huge betting sport in japan. there are obviously going to be favorites, riders who are simply faster than other riders, but there will still be the possibility that a longshot rider can sweep in and take the win.