This fighter seemed superior to anything that was out there, was it just a case of being developed to late in the war (1942-1943)? I think it could have been the top fighter in the Allied arsenal.
My understanding was it was designed to fill a specific role, that of a high altitude interceptor with a high rate of climb, I.E. bomber interceptor. By the time prototype was available that requirement was no longer any sort of priority and large scale production would only imead that of other proven types. Sometimes good enough is just that, good enough.
From Joe Baugher's usual excellent summary: "All six XP-75A long-range escort versions were in the test program by the spring of 1944. Some problems were encountered with instability, since errors had been made in the initial estimate of the aircraft's center of mass. The coupled Allison engine failed to give its full rated power. The engine cooling was inadequate, aileron forces were excessively high, and the spinning characteristics were poor. The P-75A production aircraft featured a modified tail assembly and had a bubble-type canopy replacing the framed and braced cockpit hood of the earlier versions. It featured the V-3420-23 engine of the XP-75A. The first P-75A flew on September 15, 1944. By that time, most of the bugs had been ironed out of the design. However, at that stage in the war, the Republic P-47D Thunderbolt and the North American P-51D Mustang were more than adequately fulfilling the long-range escort role, and the USAAF decided that there was no longer any need for a new escort fighter. Consequently, the USAAF decided to terminate the P-75 development program, and the production contract for the P-75A was cancelled on October 27, 1944 after only six examples had been built."