I think it's something to do with the fold. the forks and wheel are very narrow, which wouldn't be possible with a disc brake. There's one here, but you can see that the forks are well spread out, though it seems to work. I don't think i've seen a 74mm or 85mm motor with a disc fitting.
I reckon I could get my Bafang hub motor down to 100mm with some creative work on the freehub and axle, but that still wouldnt fit !
But i'm back and focused on my Fiido for now, and i've got a house move in 11 days time so everything is packed away, so wheel building it happening at work at the moment !
One side of the spokes is in, but i've gone with an outside flange exit for the spokes to make it stronger as there's quite a bit of dish on the wheel for such a small rim, but hopefully i'll finish it tomorrow.
I'll post some pics of the wheel build in progress in a bit.