It seems to have been replaced by the 2015 Challenger.
the photo above comes from my car list located here:
Max also has a list I shared there as well I highly recommend you check out! He has 11+ years in his list.
Licensing tends to very complicated. So its hard to tell the exact reason though. But, if you would like to see it get featured in a Forza title I highly recommend you vote here
https://forums.forza.net/t/dodge-challenger-srt8-392-2012-2012/558587
(was closed see Max’s Reply Below)
P.s. this does not guarantee a position on the car roster by any means.