I would imagine the reason is that the higher the seat the further back it is from the handlebars..So someone 5ft will need to be closer to the handlebars than someone 6ft 6ins..therefore a steep seat angle will push a tall rider further back and he is likely to be more comfortable..rather than squashed up close to the bars..
Bromptons at one time were often seen with a seat extender that could push the saddle closer or further away from the bars..but I do not see many now..