I think the reason might be mechanisation and mass-production. Before that, everything was done by hand, so it wasn't that much more expensive to produce interesting architecture or say hand-crafted ornaments details on buildings. Right now, costs of that would be gigantic (which is shown in giant costs of renovation of historic buildings, which needs to be done using the older methods), compared to a standard boxy builds which can be done using prefabricated components. Beautiful buildings would be much more expensive than standard glass boxes, so for the most part we choose not to build them.

Also, materials. Natural materials such as wood or stone feel so much nicer than concrete or glass.