As for the reason why, it simply comes down to quality. Turn 10 favors the delivery quality of quantity, but the studio admits that many of the tracks it had they were not happy with as they were scanned at lower resolution and simply needed to be recaptured so that can exhibit the quality and high polygon count that people are expecting on the Xbox One. For example, the Nurburgring will not be included in Forza 5, but rumors suggest that it could come later, bit Turn 10 is making no promises.
Beyond the car packs for that have been announce, Turn 10 has no news to announce about additional tracks at this time.