There is very little acceptable roofing slate available in the USA. PA has a few quarries that provide it, but according to roofers (in PA) that I talked to, it is still nowhere close in quality to, say, Welsh slate. Too many voids which when coupled with much more severe freeze/thaw cycling in much of the USA leads to early failures.

I think you may be overestimating the extent to which UK houses are held for "generations.