I think the difference between those two is due to the Post-Processing filters in game.
If I remember correctly there aren't for example multiple light sources supported by the game besides the sun. All other lights like the break lights are due to the PP filter. Hence the lights won't show up with their glow in the ksEditor. Why it works in the CM Showroom though? No idea. I guess the CM showroom has additional features added. For example, it's possible to adjust the camber or toe of a tyre (optically) in the CM showroom even when this is not supported graphically in game.
Why they didn't include the same type of filters or light interactions in the CM showroom as in the game is still a mystery to me though. Because personally I think many textures, especially dark ones look really great in the CM Showroom, even when I add additional bright light sources, while in game it appears almost too dark.