Because there's no tradition of using it for the sort of English folksongs most often performed today, and it isn't all that flexible.

I have some Scottish music for it on my website. Again, it's not much like any Scottish music you'll hear these days. It would make an interesting break in a programme but not everybody could afford the investment of time to learn something that's basically a novelty.

It woul be fine (and historically accurate) for Arne's and Dibdin's patriotic songs. Good luck finding an audience for an evening of them.