Why are @, `, and $ not used in C?

[Bjorn Engsig]

By mistake, I just typed a define as VALUE@ in stead of VALUE2, and I 
realised that it didn't look like C at all.  A quick glance on my keyboard
shows three characters, @, `, and $ that cannot be used in C outside of
strings.  Is there any historic reason for that?

I know that $ is often allowed in identifiers so that is ruled out, but
couldn't @ and/or ` have been used for something useful.
-- 
Bjorn Engsig, bengsig at oracle.nl, bengsig at oracle.com, mcvax!orcenl!bengsig