We don't use it anymore because it's expensive.
Also because no one knows how to handle it anymore and ignorant roofers will break your really expensive slate roof.
In California, because of clumsy rules around workers compensation insurance, low-skilled high-risk labor is much cheaper than high-skilled labor. As a homebuilder explained it to me: if you hire actual roofers you have to pay very high insurance rates (roofing is quite dangerous). But if you hire day laborers and tell them to install the roof, you don't. So they prefer roof designs that day laborers can install.
[https://kelly-roofing.com/what-roof-styles-are-popular-in-ca...]
Clay tile (10.8 lbm/ft2, 52.7 kg/m2)
Concrete tile (9.3 lbm/ft2, 45.4 kg/m2)
Slate - 3/8" thick (12.8 lbm/ft2, 62.5 kg/m2)
Slate - 1/2" thick (18.8 lbm/ft2, 91.8 kg/m2)
Slate - 3/4" thick (26.0 lbm/ft2, 127 kg/m2)
Asphalt roll roofing (1.1 lbm/ft2, 5.4 kg/m2)
Asphalt shingle or asphalt-fiberglass composite shingle (2.7 lbm/ft2, 13.2 kg/m2)
Metal roofing - aluminum or steel - panel, sheet or shingle (1.2 lbm/ft2, 5.9 kg/m2)
[1] https://www.engineeringtoolbox.com/roofing-materials-weight-...