I could only hazard a guess as I have no knowledge of the aircraft's certification.
My guesses might include door/structure airloads at the max gear speed, for the limiting height, perhaps Mach related compressibility problems, perhaps something as simple as the OEM's determining that there was no design/operational need for the gear to be qualified for higher levels in a manner similar to the usual F200 flap limit on jets.
Sometimes one needs to dig deeply into the design standards to tease out some of these sorts of conflicting requirements.