What about another (probably wrong) theory:
These headliners were booked a couple of years ago. Kings of Leon released a new album, and they saw how their show at BST went down and sold and thought they could be a good booking. Equally Fall Out Boy, they saw the somewhat hype around their new album and thought it would be a good booking. And then Panic! had a sort of big anticipation surrounding their new stuff, more so alongside Fall Out Boy but still.
Maybe at the time, Arctic Monkeys hadn't fully realised their future schedule and return, and maybe that was the best R&L could find.
Then they saw the reactions to the new stuff and tours from Fall Out Boy, Kings of Leon and Panic!, but it was too late to change. They would keep hoping that there would be some way for Arctics or Foos to join the lineup, but if they said they'd already booked the headliners then it might have been too late.
Equally with the new PR company, it would take time to settle in with that and maybe all the new availability of the big headliners passed them by.