Passengers with large avios accounts aren't going to be redeeming in PE.
They will be going J or going home (or some combination of the two).
Those with low reward account balances will be looking to eke out economy rewards.
So it's no surprise that PE rewards are available when Y and J are booked up even though there are few released up front (I think 2?).
For avios rewards, W is 2xY but J is 3xY. For my money J is a lot better than 1.5xW and PE is not as good as 2xY. I suspect my maths is mimicked by most of the FT community which is why you are seeing what you see.
This would be regardless of the specific departure and destination. Others have commented on why this route specifically might have more PE showing, but I suspect the general principle applies as well.