It was probably some combination of how well-integrated and how opinionated they were.
The build tool was multi-language. It allowed depending on arbitrary packages which had been imported to the Amazon package repository. It allowed package owners to annotate the packages with guidance - experimental, deprecated, forbidden (in the case of security issues), etc. You could also declare conflicts, which would notify consumers at compiletime and force them to resolve in some way. The tool deferred to a number of standard build tools in whatever language(s) you were using; it was just about getting and packaging the dependencies.
When you committed code, a build was submitted to a distributed build system. It would run your build, and then it would run the builds of every package which declared a dependency on your version. If those builds failed, your build failed (so, bump your version or make your change backwards compatible). On completion, it imported an immutable bundle of your artifact + dependencies to the deployment infrastructure.
This is the part I miss - the build stuff was great but I largely find that open source and paid options aren't so bad here. What I miss is how easy it was to manage the journey of a built package to your machines. They had a tool called Pipelines that had a visualization of this progression. Each stage, with associated machines, was linked here. The tool knew how to add stages of environments, each with their own set of deployment configurations. You could set up approval workflows: integration tests, manual approvals, etc. You could feed one pipeline into another. For each artifact, you could configure autobuilds into your pipeline, so that new versions flowed as long as tests/approvals allowed. There was support for sanity checks: if those failed, the tool would automatically rollback. In fact, if any stage fails, your pipeline would block, and you'd be notified. In some cases, a newer build that was fully functional could unblock your pipeline.
Pipelines was a pleasure to use - it really just got the hell out of my way, and nothing I've used since is as simple to integrate with.
And tying this all together, there was a tool that would allow you to initialize the end to end infrastructural pieces needed. You go to a wizard, tell it your language and purpose (webapp, CLI tool, service) and it initializes a repository, package, and pipeline with environments. Get a coffee, come back, and Do Your Job.
I imagine similar things exist at places like Google, but man everywhere else I've worked, so much developer productivity has been lost not even approximating the level of "let me do the interesting stuff" that Amazon provided.