I have not been following WEC or even Le Mans for quite some while, but when I still did I found out that Audi successfully lobbied for mandatory pitstops every N laps. The 911s would have been able to stay out one or two laps longer depending on the race track, but with this idiotic and corrupt rule in place Porsche had no chance, because a V6 does obviously have an significant torque disadvantage inside the relevant speed interval between 90 kph and 250 kph compared to Audis V10.