The RISC-V port was done almost entirely by one developer who took an interest in it. It wasn't as though the project got together and decided to prioritize RISC-V over ARM; it was just that someone did a port, and then it got (mostly) upstreamed. Nobody has taken an equivalent interest in ARM, in large part because, well, the developers are all running x86 machines as you might expect, so that's what Haiku gets developed on. If someone comes along (or one of the existing developers takes interest) in working on the ARM port more, we will hardly reject the patches!