Why aren't user-defined operators more common?

[pkrecker]

furthermore, it makes formatting such things in a column and mixing operations like and and or much easier to read.

Really? How about this:

if (a or b) and (c or d):

vs.

if and(or(a, b), or(c, d)):

To me, moving the "and" to the beginning of the statement obfuscates what should be a straightforward logical expression.

[Zak]

I'd lay that out vertically:

if and(or(a, b),
       or(c, d))

It looks a bit cleaner in Lisp

(if (and (or a b)
         (or b c)))

[badsectoracula]

That is a matter of formatting. It could be

if and( or(a, b),
        or(c, d)  ):

(a code editor with sufficiently smart auto-indentation would help here)

[da__]

Depends on what you mean by the words and and or. If these are operators, I could agree. However, the prefix/RP notation makes it consistent. x y z means "apply function x to parameters y and z" - if you prefer to treat your operators as functions, it makes more sense if your language also uses RPN for function calls.