Fibre on Demand availability

The other day someone asked me why FoD wasn't available if your line isn't served from an FTTC cabinet, even if you are in an FTTC enabled area. I know that FoD doesn't come from the cabinet, but I realised I don't actually know why there is that prerequisite.
Can anyone explain it for me please? (so maybe I'll be able to answer the question if I'm ever asked again).

In first instance it was a quick filter to cope with the exchanges where there was no VDSL2 and thus no aggregation points.

i.e. the aggregation nodes only get built in areas with VDSL2.

Rather than scaling FoD to cope with larger demand the course chosen seems to be the First First model of doing close to whole areas with FTTP. Which makes sense in terms of resources.

Thanks for the explanation. It seems a bit harsh for EO lines attached to FTTC enabled exchanges as they need FoD more than those with FTTC, but I guess it is one way to limit demand.

It also seems odd that they will build a cabinet to serve one or two dozen homes, rather than just giving them native FTTP.

Not having to buy a cabinet and its electronics, and not having to install power or pay for power usage - those would seem like big justification for putting a splitter node in the ground instead of a cabinet.