Quote from Shirtkicker :

Porsche has been in the habit of exclusively licensing their cars to single developers, which is why it's been more common to see RUF in games up to this point. I'm only speculating here, but maybe the Turbo S is still under license to some other company...

A long time ago, Porsche licensed their name to EA exclusively for a 15 year term. EA had the right to sub-license the Porsche name to other game developers, but they rarely did that. The term of the exclusive agreement expired this year (2016), and so now Porsche are able to license their name to other game developers again. Expect to see Porsche in many more games, not just Assetto Corsa. ProjectCARS have also dropped several thinly veiled hints that Porsche cars are to be included on ProjectCARS 2.

Some background: http://www.ign.com/articles/2015/02/09/ea-we-need-to-talk-about-porsche