The industry has geared itself up to produce aluminum for the mass market, and because most is made outside of Europe and where energy is cheaper, if you go for big diameter frame tubes in alloy you get a marginally lighter bike at less cost than you could achieve by hand building in light butted steel.
SS/Fixed bikes have an inherent weight advantage due to having no gear shifters, no derailleur mechs, a shorter chain, no cassette, and only one chainring. That means they can afford to build with a heavier steel frame and it will still end up no heavier than a multi-geared alloy mass market bike but also have a higher mechanical efficiency transmission, which itself is the equivalent of a pound or two weight reduction in terms of power required,. The low end geared market is still steel, and will likely remain so, because ultimately steel is cheaper to build with if you can automate it to the same extent as with aluminium. It just isn't as light, but not all parts of the bike market place the same importance on low weight.
Simplicity, price, mechanical efficiency, low maintenance requirements, and durability are the main criteria applied by riders who buy or build SS bikes.