If I'm correct, this is the first time a Turn10 staff member answers the "Why x cars which were in x Forza are not included in newer ones":
Sometimes, it's due to licensing reasons. Just because we were able to license a car for use in one or more games doesn't necessarily mean it'll be possible for another game - that's not how it works. Sometimes it is due to production realities: a matter of priority, schedule, and budget we have to work with for each title.
Cars we shipped on Xbox 360 don't take as much time and resource to rebuild for current gen as all-new cars, but they still take a considerable amount of effort that has to be weighed against adding all new cars.
We're well aware of the community's top 100 cars for FM6 (many of which carry over into FH3 wish lists) now thanks to
ManteoMax's compiled list, and we'll continue to tick those off as well as we can moving forward.