Why is Thunar the default file manager in Xubuntu?

What was the reasoning behind using Thunar as the default file manager in Xubuntu (compared to Ubuntu's default of Nautilus). Unless I have mistakenly installed it (if so, likely as another application's dependency), Nautilus seems to be installed on Xubuntu by default, but is unused.

Is Thunar more performance friendly than Nautilus? Or is Thunar simply continuing to be used because it has been the default for Xubuntu since 6.06 release?

This answer is based on historical sources on the Web.

According to this release note, Xubuntu 6.06 had included Xfce 4.4beta1 and Thunar. This release of Xfce is also the first release to include Thunar as the default file manager, replacing the original Xfce file manager Xffm (now code named 'Rodent').

This dated project site has noted the release of Thunar as part of Xfce at that time.

16 Apr 06

The first beta release of Thunar was released today, as part of Xfce 4.4BETA1.

Regarding how Thunar becomes the default in Xfce, this can be read from the history that has been written in the Rodent user guide.

[...] To distinguish Xffm as a part of Xfce, the first release was tagged 4.0.

Then came 4.2 and the love/hate story started. Those new to Xfce could not understand why Xffm had such a steep learning curve, while those familiar to Xftree expected the nerdy behaviour they had become used to.

Anyways, it was decided that Xfce would no longer distribute a filemanager: this way users could choose the new Thunar filemanager (which, as an independent software, would no longer follow the Xfce version numbers).

The short story: It seems that the original file manager was unfriendly to new users of Xfce. You can see for yourself the file managers in Xfce 3 and Xfce 4.4 on Wikimedia Commons.

Unless I have mistakenly installed it (if so, likely as another application's dependency), Nautilus seems to be installed on XUbuntu by default, but is unused.

Nautilus itself is not installed by default. Prior to 16.04 release, Xubuntu had included nautilus-data which was required by the archive manager of GNOME file-roller. The dependency has since changed in newer releases, and Xubuntu does no longer rely on such package.

Is Thunar more performance friendly than Nautilus? Or is Thunar simply continuing to be used because it has been the default for XUbuntu since 6.06?

Based on the history, Thunar becomes the default mainly because Xffm was unfriendly to new users. Another supporting reason is that the memory usage of Thunar was significantly lower than other file managers at that time.

This legacy wiki page, which was last accessible and archived in 2012, reported the memory usage of Thunar compared to Nautilus, ROX, Konqueror and Xffm.

File manager  Virtual size  Stack size  Data size  Executable/Library size
Xffm                 16568         128       4188                    12248
Konqueror            34660         128       7884                    26644
ROX                  20980         128       5544                    15304
Nautilus             27812         128       7780                    19900
Thunar               17172         128       2900                    14140

In above table, you should look at the Data size for comparison (the reasoning for this can be read in the quoted link of legacy wiki). Note that the author had disclaimed this is "by no means 100% exact" and "just a rough measurement".

TL;DR Thunar is the default file manager in Xfce, before being the default in Xubuntu. The legacy wiki had proven that Thunar uses less memory; Thunar is generally more responsive than Nautilus.