Expensive wood and fixtures. In a nice home there was lots of varnished wood, there were nice castings for hardware on cabinets and doors, lots of carved wood accents, and plenty of stone and tile. Varnished wood has to be high quality, sanded smooth, and takes a lot of wood to remove material for carving to make it look nice when varnished. Materials were heavier then, too. A 2x4 really was 2x4 and not “mill” like today, moldings, planks, decorations, trim…it was all heavier and wider. Back then you’d still need to be better off to have the nice stuff. I lived in a “normal” Victorian and I can assure you that “old world craftsmanship” was just as slapdash and unexciting as your normal home today. Hardly a straight wall or anything finer than a pine wood floor in the whole place. The old equivalent of “contractor grade” Home Cheapo finishing.

Today things can be plywood, MDF, poor-quality stitched together scraps to make trim and moldings. It’s just going to get painted, so it doesn’t matter. Way more plastic, way less metal, almost no ornamentation at all. Ply or OSB flooring with carpet or “engineered” flooring, which is often just plasticized and decal’d or veneered sawdust.

There was also no employer health care, no social security, no retirement funding or anything like that. Cost of living was cheaper. So employees and the entire production chain were cheaper. Good quality wood was far, far more abundant.

To sum up - materials and labor costs. Especially the materials. Good quality costs way more today, and then add contractor and labor costs on top of that.