The designers said the rear in particular was an important aspect because they knew many players would be using the chase cam. They probably thought the rear wing was too obtrusive on a car they designed to embody a sense of purity. So they went with a retractable wing to retain the clean profile, thought I can't think of a definite reason why they went with a standard brake light at the last moment.
I had been curious about seeing if it had customisable parts too, and I did hear rumours the winged version would accompany the one that's currently in the game. But it's no big loss.