elbear, no, there's no specific reason. We're using an arbitrary regex, 4-32 chars, alphanumerics and a set of specials including: !@#$%^&+=_
Let me know which characters you feel are appropriate and we'll happily consider implementing your recommendation. If you have any additional feedback, feel free to reach out to me personally, m<at>hackerlist.net, tel: 415.690.8033. Ultimately, we're building hackerlist for hackers, so we take your feedback very seriously.
I think that unless you have a specific reason for restricting them, you simply shouldn't restrict them, since any restrictions on the character set makes all passwords weaker. Personally, I expect all printable ASCII characters to be acceptable as part of a password and get irritated when some of them aren't accepted.
best wishes + happy hacking