A lot of premium products use high tensile steel, cars, motorbikes etc. I don't believe the Brompton frame is all high tensile steel anyway. That's why they use the brazing technique so they can use a selection of steel types each giving different properties for each part of the frame. However I think the majority of the frame is high tensile but considering the extended life this gives the frame over aluminium and the fact Brompton's cost so much you don't want to be replacing the bike after 3-4 years. Steel is more compact, it may not make the lightest frame but it can make the most compact frame that still offers decent ride quality, compared to aluminium anyway.