The problem is that Porsche are trying to make a business out of racing.
They want to sell cars to customers.
The cost of purchasing and running a competitive car in P1 is so huge (due to the rule breaks given to the oil-burners)that very few private teams would be able to afford to purchase the cars.
Given the above, it is obvious that Porsche's client base is all in P2, so why build a P1 car?

BTW - I'm Spyderman on 10/tenths