Metric sizes like 9x12, etc are not part of the ULF run because they are readily available anyway, 1/4 plate is a sheet film size as well as glass plate. 12x10 and 15x12 are historic British plate camera size.
5x12 camears were made in India and quite a few come up for sale on ebay
The roll sizes are at the request of some people making large negatives for atlernative processes, and possibly Cirkut cameras.
Ilford's sizes are based on previous demand and requests, in fact they state if you buy enough they'll cut to other sizes.