Paul is a GOP member. Johnson is not. Why would Paul even consider endorsing someone who is outside his party?
Not American, so I'm hardly an authority. It seems that Ron Paul hasn't really cared that much what his party thinks on a number of issues -- going with his own opinion over theirs. Is there any reason he wouldn't continue that policy with who he endorses (I'm not arguing that he should do this -- I have no clue who Gary Johnson is)?
Not American either but generally politicians who raise funds as members of one party from the rank and file members of that party do not turn around and endorse a rival party candidate.