Not the author, but from the FAQ[0] it says Lumina was specifically designed for the needs of TrueOS (aka PC-BSD aka desktop FreeBSD), and "works very well for the BSD community at large." It also does not require a lot of the "typical" dependencies found with more Linux-oriented desktop environments (DBUS, policykit, systemd, etc), thus I imagine making it easier to build/install on OpenBSD.
Sure. It's BSD-licensed. Given the choice for similar software, I will always opt for maximally free software. I'm not opposed to other licenses, but if possible, I choose and support the BSD and similar licenses.
In addition, it uses almost zero resources. I'm running OpenBSD on a Lenovo T520 with 12GB RAM. Everything works out of the box, which is typical with OpenBSD and Lenovo/older Thinkpads. I don't have the time to compile kernels and mess with tracking down binary blobs, which I won't use as a matter of freedom. Nothing in OpenBSD is a binary blob.
Lumina claims it needs about 150MB to run - maybe that's the reason.
I wonder how much other desktop/window manager weight (before you ask: I'm on windows 10 - my window manager eats all the RAM it can get ;-).
[0] https://lumina-desktop.org/faq/