Originally posted by: AssertnFailure
kakashi not even having veins popping out of his eyeballs lead me to believe that his is just a lesser form of itachi's
Picture of Kakashi's MS is taken from the manga whereas Itachi's MS picture is from the anime.

I cant remember if the manga showed Itachi's veins popping out.
Also the anime might show Kakashi's vein when that part finally gets animated.

Another reason might be that Kakashi does not have "Uchiha" blood in his veins.