Maybe they have other priorities than an absolutely killer, amazing and astounding artwork, and for their tastes and ideas of what a DT album cover should be, Hugh Syme is just good enough.
Who's even in charge of the thing, anyway? I mean, when it comes to discussing the new artwork, does their manager ask them "guys, it's time to think about the cover art, I'm gonna give a call to Hugh Syme like last time, is that ok"? I can't imagine any of the band members having such a strong and negative opinion on a DT artwork to the point that they put their foot down and insists with all the others, MP style (the self described "winner of arguments by wearing down the others") that they absolutely and totally need to drop a big name like Hugh Syme.