Could different chars have been used than
   c, a, d, and r?  Of course.  The c and r
   aren't needed, and we could have used f for
   first and r for rest, or h for head and
   t for tails, or | for car and . for cdr, or
   - for car and @ for cdr:

    cadaar  =  frff  =  hthh  =  |.||  =  -@--
    caar    =  ff    =  hh    =  ||    =  --
    caaar   =  fff   =  hhh   =  |||   =  ---
    caddar  =  frrf  =  htth  =  |..|  =  -@@-

    cdadar  =  rfrf  =  thth  =  .|.|  =  @-@-
    cdddar  =  rrrf  =  ttth  =  ...|  =  @@@-
    caddr   =  frr   =  htt   =  |..   =  -@@

How do you pronounce frff? .|.|? or @@@-?

One thing that car, cadr, cdr, cdar .. have going for them is that
they are (semi-)words, with pronounciation rules.  No clue how to
pronounce -some-do-hickey-@-$-> ...  which is immensly useful when
explaning thing, it is much easier to say kə.ˈdɑɹ (CDAR) of a list
then 'the rest of the first' .. or whatever it would become.