My rule of thumb:

- If there's some boring business process (that usually involves copy-and-pasting)

- Which you just know could be automated easily if only there was an API, but there isn't

- And everyone who's ever asked about an API has been told it's too difficult/expensive/etc to create an API (e.g., for an ancient legacy system)

... then there's the RPA opportunity.

In my opinion, it's not popular on HN for the same reason that legacy systems aren't popular on HN.