I think the answer is pretty simple.

It's pretty clear that Microsoft had "Everything must have Copilot" dictated from the top (or pretty close). They wanted to be all-in on AI but didn't start with any actual problems to solve. If you're an SWE or a PM or whatever and suddenly your employment/promotion/etc prospects depend on a conspicuously implemented Copilot thing, you do the best you can and implement a chat bot (and other shit) that no one asked for or wants.

I don't know Anthropic's process but it produced a tool that clearly solves a specific problem: essentially write code faster. I would guess that the solution grew organically given that the UI isn't remotely close to what you'd expect a product manager to want. We don't know how many internal false-starts there were or how many people were working on other solutions to this problem, but what emerged clearly solved that problem, and can generalize to other problems.

In other words, Microsoft seems to have focused on a technology buzzword. Anthropic let people solve their own problems and it led to an actual product. The kind that people want. The difference is like night and day.

Who knows what else might have happened in the last 12 months if C-suites were focused more on telling SWEs to be productive and less on forcing specific technology buzzwords because they were told it's the future.