I'm going to guess that had Wales still had ample tree cover 100 years ago that most of it's housing with have ended up with wooden shingles too. People use what is cheap in their local area to roof their houses. Or at least the used to, before modern transport arrived.

Which also leads to oddities in some places, like Dorset, where you have houses roofed with substantial bits of stone that threatens to fall through the inadequate wooden sub structure at any moment