Bowser has a really bad temper, a sulfurous body odor that'll make your eyes water and breath that'll turn your hair white, so he doesn't like to interact with the public in person.
Which is why he had to hire his assistant Kevin, well, that and because he's so obsessed with Mario that he never gets any work done. Kevin has figured out how to streamline and improve upon Bowser's operation, which is great news for Bowser and not-so-great news for the Super Mario Bros.