Because Revolvers fit the hand better, point more naturally, as a class , are twice as accurate as semi's .
I Won't pull out the reliability thing . Revolvers CAN malfunction . BUT , semi's have a longer check list of prerequisites to reach the same level of reliability as revolvers .
Traditionally, Power ! was an advantage of revolvers . Today, it's a little more ambiguous , least ways with some parameters along the lines of more or less belt carry- able , and reasonably controlable , and not very painful , for a fairly competent shooter . i.e. disregarding Desert Eagles , Automags, and X Frames as outliers .
The 4in medium frame .357 mag is still the handgun with the widest amount of absolute versatility . But .357Sig , .45Super , and full power 10mm all reach very credible levels of power and performance .