I think the timing really didn't help, it was so close to Christmas, it was actually cheaper than the Arena tour, but with a lot of people having spent a lot of money on the Arena tour, more casual fans are going to be less likely to part with £60 to see Muse in 6 months time so close to Christmas having just seen them.
I offered a spare ticket I had bought to my friend when I got the Etihad ticket back in December and he said that if it is still available closer to the gig he will definitely go but he just couldn't justify it at that time. I'm sure a lot of people were thinking the same, but unfortunately they may have lost interest by the time the gig came along (and they didn't have me badgering them to go).
I personally think Muse are great value for money, especially the stadium shows, they really are spectacular. But it is still a lot of money, £60 for ticket, plus travelling to shows, the food and drink isn't cheap (they don't let you take stuff in), might grab a t-shirt - it soon turns into a £100+ per person outing.
Having said that I did think they0 show at the Etihad last night was pretty packed despite it not being a sell out, in a way the fact that it didn't sell out has it's advantages as I'm the touts are not having quite as much luck flogging tickets and ripping fans off!