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

[Steve Summit]

An amusing bit of trivia is that pcc (4bsd, at least), among
other heroic efforts at antediluvian backwards compatibility,
still "accepts" (if only so it can reject it) an old BCD constant
syntax involving backquotes:

	$ /lib/ccom
	int x = `1234567` + `8;
	line 1: BCD constant exceeds 6 characters
	line 1: newline in BCD constant
	line 1: gcos BCD constant illegal