It could be a cultural/religious thing. You wouldn't ask a Sikh why he was still wearing a Turban 36 years later. Or at least I hope you wouldn't :p

Having a mysterious group/cult with an agenda (that turns out to be aiding Palpatine) has the potential to work really well, even if it is subtle and in the background.

We might assume they're the "Sith" race, of which Palpatine is also a member (this would mean pushing the EU aside, of course). We might see them as an influential secret society working in the shadows, surreptitiously manipulating Palpatine's rise to power from behind the scenes, like a galactic Freemasons crossed with something out of the mind of David Icke.

Actually, it might suffice to insert them into the background in particular significant scenes that involve Palps. If you could go back through the Prequels and discover more scenes like the above picture where they're seen watching in the background, it would be a cool easter egg that people might not spot in their first viewing. I'm thinking along the lines of the G-Man in the Half Life series - a sinister agent of unknown origin who manipulates and watches over the character and no-one knows who he is.