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In the TeXbook, Knuth asks:
Exercise 7.2
Can you imagine why the designer of plain TeX decided not to make
\\the control sequence for reverse slashes?
Is it something to do with the fact that control sequences formed by the escape character followed by a non-letter are control symbols and they expect a letter to follow?
Knuth answers this in the TeXbook code (lines 2,300-2,305, or p 307):
\exercise Can you imagine why the designer of plain \TeX\ decided not
to make `|\\|' the control sequence for reverse slashes?^^{backslash}
\answer Reverse slashes (backslashes) are fairly uncommon in formulas or
text, and |\\| is very easy to type; it was therefore felt best not to
reserve |\\| for such limited use. Typists can define |\\| to be whatever
they want (including |\backslash|).
or formatted:
Q: Can you imagine why the designer of plain TeX decided not to make
\\the control sequence for reverse slashes?A: Reverse slashes (backslashes) are fairly uncommon in formulas or text, and
\\is very easy to type; it was therefore felt best not to reserve\\for such limited use. Typists can define\\to be whatever they want (including\backslash).
\answerReverse slashes (backslashes) are fairly uncommon in formulas or text, and\\is very easy to type; it was therefore felt best not to reserve\\for such limited use. Typists can define\\to be whatever they want (including\backslash).\\has a definition when Plain TeX starts off:\meaning\\printsmacro:#1pt->#1\\has a definition whenplain.fmtis loaded has an unexpected consequence.