'Dark' doesn't mean dark, it means dark!How is it that a Pokemon's family that is basically themed on light and illumination inappropriately typed by not being part dark? It doesn't even have any of the usual dark-type tendencies.
...okay, hold on, scratch that...
Luxray could've gone either way. In this particular instance, it's wrong to firmly say that it HAD to be one typing over another. If I had to wager though, I'd say most people are like me and would've liked to see it become an Electric/Dark typing in order to get at least one Pokemon with that combination out there. There's been plenty of pure Electric Pokemon as it is (and they like to run with Crunch of all things too).
Plus, Luxray looks pretty wile and ferocious. It will fuck yo' sheet up, mang.