Go and read the thread yourself [1] and you will see that the reason it was rejected was that no one saw a reason to take on the maintenance burden when you already need to have Gnu Core Utilities in ports to be able to build a heap of other ports. Focusing on theo@ calling out the submitter for having their judgement clouded by "Because Rust" misses the point.
[1]: https://www.mail-archive.com/ports@openbsd.org/msg143875.htm...
Besides, Rust already has a solid presence in ports and I know several experienced OpenBSD developers dabbling in it. But the language will never make it into base in its current form as OpenBSD expects self hosting and I think everything in base is Posix(-like), apart from Perl, but I have not taken the time to go digging through my /usr/src.