I truely think that Porsche didn't enter Motorsports again yet because they have the Formula One on their mind and they know that right now, it is impossible to win against Ferrari, McLaren Mercedes and BMW.
Maybe they wait for the right moment, when Ferrari gets bored of winning, when Mercedes gets bored of investing so much money and when BMW already won one title and maybe return back to baking homemade cookies.
The only international race class which makes sense to race in (from a prestige point of view or better said: from a marketing/advertising point of view
) is the Formula One.
And racing the Formula One not only requires the skills but also a lot of money. Probably Porsche also wants to avoid some embarresment at the beginning, especially in the first year.
That said, I don't think that Porsche will return back to high class international motorsports, maybe an event here and there but not something really important. Sad.