I don't know a ton about the world of professional racecarring, but I do know that Porsche is sort of a big deal when it comes to that stuff. Hell, there was an entire "Need for Speed" game dedicated to the brand. And therein lies the problem. EA has an exclusive licensing agreement with Porsche which allows them, and only them, to have Porsche cars in their games. In the past, EA has been willing to sub-license the Porsche brand to other game makers (as was the case with the last few "Forza Motorsport" titles), but apparently this year is different. In a FAQ on the official "Forza" site, creative director Dan Greenawalt, stated that Porsche would not be in this year's "Forza" because EA wouldn't let them.
"As many of you know, since the beginning of the Forza franchise, the Porsche license has been available only through a sub-licensing deal with EA. In Forza 3, we were able to feature more than 35 different Porsche models by offering to collaborate with EA. For Forza 4, we were looking forward to adding even more Porsche cars, and we were especially looking forward to featuring multiple Porsche experiences in our new Autovista mode. In the end, however, EA couldn’t see their way towards collaborating again."
Much of this likely comes down to the fact that EA has a racing game coming out shortly and the two games are direct competitors, releasing in the same holiday time frame. "Forza 4" drops on October 11, while "Need for Speed: The Run" comes out on November 15. It's possible that EA plans to promote their exclusive access to Porsche as a reason to hold off and pick up that game instead. Granted, the two titles are very different racing experiences, but in the crowded holiday season, companies often adopt a take-no-prisoners approach to getting ahead of the competition. They're certainly in their rights to do it, but it does tarnish the "Forza" experience a bit in the process.
It's not the first time a major racing game has had to skip out on the Porsche brand, by the way. "Gran Turismo 5" doesn't have any Porsche cars either, for the exact same reason.