Ummm... Another simple reason? Especially when used with a silencer, it's the most quiet... which (to state the obvious - please don't rag on me...) may be VERY advantageous for an assassin in some circumstances.
What did Mrs. Bullet say to Mr. Bullet? ... "We're having a BeeBee!"...
IF THE SHOE FITS, WEAR IT!... IF THE GUN FITS, SHOOT IT!