Manufacturing capacity plays a part I'm sure. The overseas suppliers are Al specialists because it's been the primary material on the market for such a long time. The same children who weld all aluminum dual suspension bikes can weld the aluminum hardtails, switching to steel changes the game significantly from a manufacturing standpoint.Its definitely nice to see some budget friendly hardtails out there, but I am a little surprised there's not more steel hardtail options. There seem to be a fair number of UK brands with steel hardtail options, but not many others that I am aware of (well, other than Chromag of course). Not to be that over-the-top "steel is real, brah" guy, but they do ride noticeably better and I would always take a steel hardtail over an aluminum frame, even with the weight penalty.
Al is a bit more forgiving when it comes to tubing, to make a light steel frame you need a decent quality tube-set, to do the same with Al you can just use whatever's laying around. There's probably also 10x as many parts and pieces like BB shells, head tubes, and drop-outs available off the shelf. When trying to make a $1200 bike, that stuff adds up fast.
A nice budget build steel hardtail would be pretty sweet though.