Because Toolbox is a Red Hat/Fedora project and Distrobox isn’t. Also, Distrobox is a much more recent project (2021) compared to Toolbox, which was developed back in 2018. When Silverblue came out, there was a need to make it easier to install apps, and thus Toolbox was born.
Since Toolbox is a Red Hat/Fedora project, it means that it’s officially supported, whereas Distrobox isn’t. Not that it means much from a community support/home use case of course, but that might matter if you’re an enterprise and you want support from Red Hat or official Fedora communities.
But both use podman behind the scenes so internally they aren’t that different, but you can think of Distrobox as a more distro-agnostic and user-friendly version of Toolbox. If you’re a home user then stick to Distrobox.