The would-have-been speaker's blog post addresses this question specifically. He apparently pushed back many times on the idea of giving the keynote on his work and was assured that the Rust organization was aware of the topic and wanted him to go ahead [0]. Then they walked back on that.

That's what I think most people find terrible about Rust leadership's handling of the situation. It's not that this specific keynote was super important, it's that the team let the speaker spend a few weeks preparing a keynote and then pulled the rug on him.

[0] https://thephd.dev/i-am-no-longer-speaking-at-rustconf-2023