Isn't it likely that the reason she hasn't done other visits is because they're arranged further in advance? They seem to be announcing things she's doing very very shortly before they will happen.
Wimbledon is happening either way, and if she can't show up, it's not a huge deal to get someone else to hand over a trophy.
A visit to a charity must involve a reasonable amount of inconvenience, and then potentially be cancelled at short notice the day before if she's not up to it.
Her appearance at Wimbledon can also be considerably reduced if necessary - no need for her to watch the whole thing, she can just do 5 mins of a trophy ceremony. Harder to do that with a charity visit I guess.
As I said, I'd happily abolish the monarchy. But I don't have anything against her specifically, nor do I have an issue with someone with cancer being a bit choosy about what they do.