My guess is heat. The 7800x3D only consumes 85W max, but cools like a 120W chip thanks to the extra cache acting as a thermal blanket. The 7940hs has a wattage of between 35 and 55. That’s gonna be hard getting a chip to perform at max under 55W so it can still be cooled like a 55W chip. With the max clock speed hit such a wattage will cause, the 3D cache may not make up for that performance hit.
My guess is heat. The 7800x3D only consumes 85W max, but cools like a 120W chip thanks to the extra cache acting as a thermal blanket. The 7940hs has a wattage of between 35 and 55. That’s gonna be hard getting a chip to perform at max under 55W so it can still be cooled like a 55W chip. With the max clock speed hit such a wattage will cause, the 3D cache may not make up for that performance hit.