The listener, as with several other outlets, tries to stick with a first in first out basis. Of course if there is a date involved that will be taken into account. It just so happens that a number of setters have produced carte blanche puzzles at the same time, not to increase difficulty per se, but because their themes warrant it. It is not the editors fault. I would not like it if my puzzle was delayed several months just because of the layout. I am sure the editors would not allow a puzzle that is carte blanche just for the sake of it. I suppose Phi could have added the bars in the bottom two-thirds of the grid, but that is a little messy and, as mentioned, it is the Listener after all.
If anyone wants to tackle an even tougher puzzle then try this month's Magpie - a circular puzzle that took me a lot longer than this puzzle did