Why is toolbox preinstalled and not distrobox?

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.

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.

I'm a Fedora user and Distrobox just seems more complete as a project and the commands make more sense to me

I stuck with Toolbox for a long time because it was default, but then I wanted to be able to easily recreate my *boxes with the same set of packages when e.g. they broke for some reason, or because the distro they were built on released a new major version. Distrobox supports that with its assemble command, so I switched. Otherwise it's not too different really, for a casual user like me, and if I hadn't needed assemble, Toolbox would've been just fine.

(Except that I keep forgetting whether Toolbox or Toolbx is the correct spelling now.)

I use Distrobox on Fedora Silverblue.
More precisely, uBlue. It came pre-installed there and I quite like it.

Toolbx is more of a "use it to install command line dnf-packages on SB", while Distrobox is way more capable.

I can have any distro I want as container and export graphical apps and binarys.

All of the universal blue images come with distrobox so I gotten used to that, it's nice that you can export apps so they appear in the DEs application menu

Can I ask why you choose to use one of those weird "immutable" distributions in the first place, out of curiosity ?

For general usage, it doesn’t really matter. Distrobox is inspired on toolbox and provides some added functionality and configurability, like init scripts and the ability to run different distros, as well as creating desktop shortcuts on your host system. If you don’t need all of that, I’d stick with toolbox, as it’s preinstalled and works well.

Distrobox was always stable for me. Autocomplete only in bash but that doesnt matter much. Waaay more images by default but not as curated, also many are maintained by Fedora people and not the Distrobox people, so its not like they actually support more but just ship.

This is a big difference, Toolbox also supports these images.

But featurewise distrobox is brilliant, love the app icon export, the binaries are maybe a bit bloated.

Why not both ? Toolbox is the fedora/redhat solution, which is the why, and makes it the choice when something's in the fedora repositories, or if you want to trial it before (considering) rpm-ostree install, but an Arch distrobox gets you the AUR, not to be sneered at...

I use toolbox: Distrobox is a pretty horrible shell script and deleted parts of my home directory when I tried that.

In the end I just pointed toolbox to a script named podman that just adjusts the setup to what I need, implementing the missing features I wanted that way.