Although I don't know the engine mount position in the P-59A, I had already had the thought that a P-39 / P-63 was an excellent choice for the preliminary testing because of the rearward mount of the Allison in the two Bell fighters. Changing the MAC toward the rear was going to be a problem for designs with forward mounted engines, becoming hugely nose-heavy.
Chances are pretty good that the P-59A's engine was enough lighter than the recip usual, that longitudinal balance, whatever it's called in aviation engineering, was a potential problem, and looking at the P-80, it looked like a lot of extra nose sticking out in front of the pilot that wasn't needed, if not to provide additional forward moment arm?