*pop*

In all seriousness, the reason is that in the early days of the Thumbpire, nobody would listen to them when they spoke with their proper accents. They started to affect a more cultured sounding accent, and suddenly planets were falling over themselves to be dominated. Now, it's just the way it's always been done, since none of the officers from that time survived.

Of course, Black Helmet Man doesn't need to do this, because his voice is amazing.