It's a combination of things really. First, the space here is finite, so I have to be very choosy about letting any of them sit. Then I want to give every hen that feels the urge the chance to raise a brood, so those two reasons together mean they have to take turns. Typically a pullet would be jumping the queue, and being British, we frown on thatI'm sure you've mentioned it before, and I'm sure your reason for doing so is very good, but why don't you let the under 1 year olds hatch?
For maturity reasons, like more of a chance to break, or over commit?

Fez is a few months older than Idris, and has already been broken once, and since her elders either have had a go at least once (e.g. Maria), or have shown no sign of wanting to, I thought if Fez went broody again, I'd let her have a go next. And that's where we got to about a week ago; she's been sitting 6 days now.
So Idris will have to wait; how long depends on how successful Fez is, whether we get any unexpected losses in the flock, and whether I manage to rehome any of the older birds (there is another potential rehoming in development). Hopefully she won't have to wait too long.