Because 5 and 6 GHz is basically the same band. 6GHz is just an extension of 5GHz. There is only one physical radio set serving it. Just like your car stereo can be tuned to both 88 and 108MHz but not at the same time. Theoretically, you could have two radios for 5 + 6 but that would be the same as having two routers. 802.11ax is specifically made to improve the co-existence of multiple devices on same band (with BSS colouring etc.) so having two radios for same band is kinda pointless.
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