The 2nd Wembley didn't "properly" sell out (I say "properly" in case anyone goes on about technical sell-out what the promoter wanted to sell blah blah blah) - there were some rows of seats covered over near the top of the upper tier which were sold on the first night (which was rammed). They did set an attendance record the first night, from memory it was something like 88k first night 75k second. Still massive numbers.
As for slow sales, maybe the promoters are finally being bitten for the endless price rises and (though I like it) it's fair to say The 2nd Law hasn't really connected with the masses. It seemed at the Emirates that bar Supremacy people in general weren't that responsive to it.