Why do most single propellers have a clockwise rotation?
I wasn't able to find any good historical references, so this answer is pure opinion.  (sorry).  I'll break this down into two separate questions.


Is there any reason for single-engine propellers to turn the same way?


Yes.  When flying, the spin direction of the prop has several effects.  Helical prop wash, p-factor, gyroscopic precession, differing blade angles of attack all are due to the asymmetry of spinning in a single direction.  An experienced pilot will want to understand and compensate for these effects when necessary.  It is easy to see that once this is understood, a single configuration will be preferred within a community to minimize differences between aircraft.


Is there any reason to prefer CW rotation over CCW as a convention in a single engine plane?


None that I am aware of.  It appears to be simply an accident of choice.  I had wondered if the majority of engines available to early manufacturers was in that direction and made it preferable, but I couldn't find anything suggesting that.  Indeed today the majority of automobile engines rotate the opposite direction.  Making (piston) engines rotate the other direction is not a huge problem.  Twin-engined aircraft and watercraft often have counterrotating engines and propellers.