To be fair, they have been playing these size of venues for a while now. They have done smaller venues here and there though. I dont doubt they will do realtively smaller venues in London for example, similar to what Blink 182 done, in the future. However, i miss the days of venues that werent even Academy's let alone Arena's!
Personally, i cant complain too much, i saw them at the Corn Exchange which is a relatively small venue compare to these current dates last year. In which it was one of only 2 dates (that werent a festival) at that time. Swings and roundabouts.
Theres probably numerous things going on behind the scenes we dont know about! For example, maybe the promoter didnt offer them a date to play in Scotland. There is really only one promoter in scotland that would be able to front the cash for a show of this scale (DF Concerts).
Who knows! Plenty of empty dates in the tour schedule for things to be announced!