Distrobox is directly inspired from Toolbx and was created because of limitations of Toolbx and how Toolbx’ maintainers didn’t want to implement some features at that moment in time.
Currently, Distrobox is almost a superset of Toolbx. Though, I’ve come to the understanding that Toolbx does better at some tasks.
If you would like to stick to just one of them, then Distrobox is probably still the better one and should be preferred. However, if its added functionality doesn’t do it for you, then please feel free to continue using Toolbx.
Why is toolbox preinstalled and not distrobox?
Because Toolbx predates Distrobox and is developed by developers that are associated with Fedora and even specifically designed in hopes of solving some issues pertaining to Fedora’s Atomic distros.