Image (c)2017 SandTEngineerJust to recap where those windows went, using this picture of SandTEngineer's - one of few that shows the middle of the train.
The ends of the 26 m carriages are tapered off to thread them through curved platforms, and - to limit the stepping distance - the doors have to move in from the ends as well. The pocket for the doors to slide into thus goes towards the centre, where the side wall is flat, with no windows there or on the other, slanted, side. Now none of those design choices is absolutely dictated, but as a designer you wouldn't do things the difficult way unless someone gave you a strong reason to.
The ends are occupied only by storage and toilets (plus a baby change room - important if you suffer from wrong baby syndrome). But while some of the passenger cabin ends by the doors are also storage, half of them end up with seats.