I'm not claiming that such a port would be easy, but that the Raspberry Pi represents a relatively stable platform. Rather than having people turn away when they discover that component X of their PC is unsupported, they have a path where they can boot the image, try it out, and learn how to develop software for it. While this probably wouldn't address the issue of kernel/driver development, it may help to address application development.
The problem, as others have said, is the lack of dedicated manpower. This is one of those situations where I wish I was a developer, I'd dive right in and try to get Haiku to daily-driver stability on the platform. There is work being done on porting to ARM already, and my hope is that as the X86/64 platform matures, focus can shift to ARM and the Pi specifically.
The future, rather than the past.