I don't think much of it comes into play besides the first and last,There is always something else on,Hurling matches on at the same time etc! most likely just to do with the recession!
Many people who attend these big outdoor gigs attend plenty other big music events in Ireland and Europe every year and have to carefully decide what to attend and what to skip. The possibility of an Oxegen and Slane announcement when the Croke Park gig went on sale would not have helped sales.
Then take into consideration that 135,000 people attended Phoenix Park over 3 days just a week after the Croke Park gig.
I was at RHCP O2 gig and went to all Phoenix Park gigs.
When the Croke Park gig was announced I decided to wait and see what other big summer gigs would appear.