Just to clarify, you mean there's no (official) build for ARM/MIPS/etc, right?

DragonflyBSD itself is officially supported on x86_64.

The only good reason I can think of to support something other than x86_64 is running on something like a raspberry pi or some sort of 'embedded' device. That's not really suited to the goals of Dragonfly anyways, NetBSD or Linux are a much better fit there.

Well yes because Dfly has really small community compared to even FreeBSD .. supporting/testing multiple architectures is a monumental task for distro maintainers.