Good summary here from SkySport F1
These tyres are extremely temperature sensitive either side of their operating window; just slightly too cool or too hot and they lose whole chunks of performance. The Ferrari - and the Red Bull - can evidently keep them in this narrow window more easily than can the Mercedes - and this is almost certainly driven by their respective aerodynamic philosophies as the cars work in very different ways aerodynamically.
That was the case last year too but the Mercedes team quickly understood how to operate its car very effectively to keep it in that narrower operating band. This year that tyre temperature sensitivity has increased - partly because of the higher fuel consumption everyone is suffering. The cars are using more fuel this year for two principal reasons: a) they have more downforce which means more drag and more time at full throttle and b) the seasonal three-engine limit (down from four) means the engines are being run slightly richer in order to protect them. Engines can be run closer to the detonation threshold with a weaker mixture (less fuel/more air) to give more power, but it extracts a cost in durability.
When drivers have to back off to bring fuel consumption on schedule, it can bring a further problem in taking the tyres below their temperature window threshold. If the tyre has not much tread left, it can be impossible to then bring it back up to temperature. This, of course, does not explain Merc's tyre difficulties in qualifying, but could be contributing to it in the race.
