I think because there's no indication any of the other Masters were hearing drums. You'd have thought they'd have mentioned it at some point.
Maybe it wasn't as severe back then. He didn't mention that he was lactose intolerant either.
Of course it's a retcon, but it adds a little something to the character to make him something more than just "like the Doctor, but EEEVIIIL", so that was a welcome addition to the character, as far as I'm concerned.