Probably licensing issues. Ask History Channel?
Update:
Right now since at least Jan 2021 the Canadian History channel has already started to offer it, but it is a longer version different from both amazon and somewhere else. The Canadian Ep 1 is 93 min while the other two is 84 min and 80 min respectively. (Not sure about the third's official source bc I saw it on some Chinese web but for the Washington 2020 an uploader tags it with "hulu" so I guess it is the same for Grant 2020)
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Top Gap
What is the broadcast (satellite or terrestrial TV) release date of Unlikely Hero (2020) in Australia?
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