is why...
As a Linux user, I had been looking for an BBC Micro emulator. At the time, b2 and beebjit were not on the scene so the choice seemed to be mainly between BeebEm and B-Em. I noted that the last version of BeebEm for Linux seemed to be well behind the Windows version. Interestingly, BeebEm started out as a Unix application but when someone, and I think it was before Chris, the current maintainer, ported it to Windows they didn't make it cross-platform. Later, some of the Windows changes had been back-ported but there was still a big gap. By comparison b-em had the source organised so the OS dependencies were isolated into a few modules and the biggest dependency was on a library called allegro which was, itself, cross platform.
So, I choose B-Em for my own use. I wasn't specifically intending to maintain it but it ended up that way.
I agree with Chris that to make it cross-platform would be a lot of work but, if you fancy that, then go for it. I'd also say do take stock of the the emulator "market". As I said, beebjit and b2 are new since I started work on b-em. I suspect both are more likely to work well on a Pi, though b-em still has advantages if you want to emulate more things from the ecosystem including tube processors etc.
