The UK architecture minister (they have an architecture minister??) said that the scheme as a whole was just becoming too expensive. Apparently it all rested on putting a road underground nearby, to the tune of about £500 million. The building itself was a mere £65 million in comparison.
Now though the government is proposing a temporary visitor centre instead, which seems like a good way to build something shoddy that will not function very well and will probably cost them more in the long run anyway (or hang around for longer than it was ever meant to, like those classrooms in Victorian schools built in the 1950′s that only had a 25 year life-span but are being vainly renovated to this day).
DCM are understandably a little bit peeved.