I'm so confused why their Cloud offering is so different than their Bitbucket Server.

Because Bitbucket Server had syntax highlighting probably for more than a decade now.

bitbucket server is a very different product. It's not the cloud version that's been containerized.

Atlassian has a separate tool chain, FishEye and Crucible, for code reviews. It's a separate product with a separate license and very few people seem to use it, but the Atlassian suite is meant to be a suite, as in many separate tools for specific purposes, instead of Gitlab's approach of having everything built into the core product. Atlassian's actual code review tooling is just as good as Gitlab's and really better because you can use it across arbitrary repo sources and don't actually need your code to be in Bitbucket or in a single repo or a single server at all.

You're just not using it if you only use Bitbucket.

Why invest a lot of effort this year in a “new pull request experience” then? https://support.atlassian.com/bitbucket-cloud/docs/try-the-n...

For what it's worth, we use the Visual Studio Code extension for reviewing pull requests ; it comes with syntax highlighting and it's more performant than a web-based interface.