Strongbox was designed prior to AWS Parameter Store, and took its inspiration from projects like Fugue CredStash and similar AWS KMS based projects. While there are certainly pros and cons with the different approaches, which you can see in the linked comparison table, I think one of Strongbox' strengths is that it is easy to set up, and you get a lot of convenience functionality related to secrets management. Not having to maintain running services, and natural compartmentalization using both AWS IAM and AWS accounts was two of the drivers.

$1 per group of secrets, e.g. a service. In many cases I think this is fine, but it is certainly not a strength.