I have a two-part theory about this. 1. Ford was pushing their rear wheel drive/four wheel drive hybrid drive train on the police version of the Explorer. But, they've dropped the 3.3 liter V-6 hybrid from civilian Explorers. If they are maxing out their manufacturing capability in that market, they may not have parts for us civilians. =OR= 2. Ford isn't confident about the reliability of their hybrid drive train which is why they've dropped it from the Explorer and aren't offering it on the Expedition.
The hybrid drivetrain for the Explorer is different from the PowerBoost though. For the Explorer it was - according to Ford - about production capacity where they couldn't make enough due to high demand for the police version. Ford then
choose to not add production capacity, as they believed they could skip civilian right to full battery-EV. At least that is what seemed to have happened, as nearly no investment at all occurred in both Europe and US with plug-in hybrids, but all chips going to full BEV. That dramatically backfired as we know, so now they are scrambling. So I don't think it is reliability. Also as odd enough they do sell a hybrid civilian Explorer in Europe (Explorer ST with plug-in, same setup Lincoln had), which also seems to underline that it isn't about that it couldn't work.
But for the Expedition they have the capacity to build it, as that hybrid is widely available on the F150. As far as I know, no reliability issues? Note that this is a completely different electrical-engine and battery setup, and a bold-on solution to the existing gearbox. Here the issue is just that the Expedition is built in a different factory and hence that assembly line needs to be converted to support it. Union contracts seemed to suggest they were going to do that, but apparently not just yet now.
All in all, disappointing IMHO.