The actual reason is simple. Brompton try not to use standard off the shelf hubs so you are forced to pay more by buying from them. As the majority of Bromptons are used for shorter distances its not economical to have an 8 speed hub designed specifically for them. There are people who modify the bike or hub to create an 8 (or more) speed version but Brompton don't like it and will blacklist these people from buying parts from them if they find out.