Many many of his books were best-sellers, and remained so for a long time.
Plus I suspect a lot of parents had them around the house, and they've found a second life in their children.
The covers always fascinated my child, for example. I have the complete collection of Discworld novels as paperbacks, and I remember my kid looking at them, pulling them off the shelf to do so, even before he could read.
In short Pratchett was very much mainstream, in the UK for a long, long time.
I've always assumed to be non-mainstream, so I'm surprised but happy to see how he seems to move into the list of cultural eternals.