Two reasons actually:

1. After getting really familiar with EndeavourOS, I was just curious about how hard an actual Arch install was. Then I found out about the official archinstall tool bundled in the ISO, decided to try it out, and it gave me a relatively barebones KDE desktop that was super snappy and that I could expand however I wanted. It just felt nice so I decided to stick with it. Now I am so used to using archinstall on my many Arch deployments (desktop, DIY NAS, home theater PC, work laptop, Surface Pro 7) that it really just feels like home.

2. After Antergos shut down, I briefly used Anarchy installer. When that also shutdown, I became a bit wary about the longevity of these smaller community-driven Arch-derivatives. I don't have anything against them and it's super cool that these projects exist to expand the appeal of Arch to more users, but personally I wanted to be familiar with something that I knew would exist for a really long time and wouldn't close down due to the developers getting too tired of doing maintenance, which is a very real thing in FOSS. I am constantly getting new devices and installing Arch on them, so finding a more permanent solution that I knew was always going to be there was important to me.