I think it is a mix of changing taste in architecture, costs to using older materials like thick stone, labouring costs etc., and newer materials that made people move away from the style of architecture we used to see and cherish so much to preserve these days. Imagine what it would cost with the cost of labour to use those large stones they used to use instead of modern materials. Then there used to be a lot of craftsmen who did all the ornate finishing touches such as ornate stonework, moulded plaster ceilings, and ornate timberwork which these days would be super expensive but in those days the labour and timber was relatively cheap.
Modern materials such as readily available steel, aluminium, modern large sheet glass, plasterboard, veneers, plywood, plastics etc. gave architects the ability to make radical new designs for buildings allowing for bigger internal spaces and even weird shapes as well as taller buildings that old style building materials could never be used for, and generally for the size of the building could be built way cheaper than with older materials.
I agree with you though that these new buildings rarely have the charm and beauty of the old stone architecture. New buildings may be functional but few have the solid beauty of the old buildings. Many old buildings in the shopping precincts had their ornate fronts removed in the 1960s and 70s and replaced with modern sterile aluminium and large plate glass. You see some of the pictures of those buildings and see the beauty they used to have, and now they are bland and sterile. What were the architects back then thinking. Some towns who resisted the modernising of their buildings and kept their old world charm seem to now have become tourist towns and are booming because they have cleaned up their old buildings and restored them to their former glory, and people flock to see the beauty that craftsmen put into these old buildings. Back in the 1960s and 70s many saw the old buildings as an eyesore and modernized them or tore them down to rebuild in a modern style but many replaced them with plain sterile buildings that have no charm.