[Mod Edit - Abbreviated profanity, profanity and profanity that is disguised but still alludes to the words are not permitted - D]
This has been explained a number of times.
At the time, T10 didn’t have the rights to Porsche because EA had them. Same with previous Forzas where it’s been either added as DLC or not added at all.
For FM6, you may remember them added to FH2. This was the only time T10 had access to the Pirsche name, so it was added to FH2 promptly. The reason it has yet to appear in F6 is because 1 car takes 1 team 6 months to model to the required standard - thus it takes a hell of a long time to do a whole manufacturer list. More time than T10 had before F6 was released, thus it is going to be added in early 2016.
Patience is a virtue.
Edit: read ManteoMax’s thread on Porsche here: http://forums.forza.net/turn10_postst37756_Porsche-and-FM6--update--official-press-release.aspx