Physics says that the AIM-9X will have inferior kinematics (speed and range) and a smaller warhead than its competitor.

The AIM-9 is a 125mm (5″) diameter missile, while its competitors, the ASRAAM, Python 5, IRIS-T (I think), R-73 (AA-11), MICA, are all 150 mm+ (6+ inches), giving more than 40% internal volume.

This means more fuel and a larger warhead.

Additionally, when compared to the other thrust vectoring missiles, which have new motors which provide less thrust during the early thrust vectoring stages, and hence less thrust vectoring losses, the motor is less efficient.

There is also the argument that with an agile airframe, and a seeker that has a wide scan angle, you can get the same effects, which is what the Python 5’s game, and/or using the capability of locking on after launch (ASRAAM largely operates in this mode) to achieve similar targeting, because all these missiles require that the platform pass pretty detailed information on the target, typically with a helmet mounted sight, to the missile in order to engage off boresight targets.

None of the larger missiles even fit in the F-22 side bays for IR missiles, and once one is in a visual encounter, the F-22 has lost a its advantages of stealth and super cruise, so the USAF probably sees this as a weapon of last resort (the cannon is more for that random enemy truck convoy that happens by).

That being said, this is a neat video.