Because he's had to pull out of public engagements so needs an explanation maybe? We're not suddenly getting information; he also announced the routine prostate surgery before it happened and that wasn't serious, so maybe he's just decided to be more open about these things than the Queen was.
Along similar lines, I doubt Harry found out about the cancer at 6pm last night like the rest of us, so I'm not sure why the press are implying he's rushing over on an urgent flight as if Charles is on his death bed. He probably knew about it days ago and of course you'd go and visit your parent if they had just been diagnosed with an illness like cancer. I don't think it would stand up in court as evidence that Harry wants to make sure he's in the will.
These people obsessed with hating Harry and Meghan are embarrassing - they are genuinely infuriated by the mention of their names!
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Increase awareness of an unknown cancer