I wouldn’t say that it never caught on. I run a feed reader and ~6% of feeds have WebSub. Most of these are probably wordpress.com blogs which include it by default.
YouTube also sort of supports it, but they don’t really follow the standard so I don’t think it counts.
But the nice thing about WebSub is that it is sort of an invisible upgrade to the existing feed (or any other HTTP URI) so it just works when blogs enable it.
Most major feed reader services support it. One problem is that you need a stable URL to receive the notifications. So it is hard to make work with client-side readers. But I don’t think there is really a way around this other than holding a connection open to every feed you follow. So I would say that it does its job well. I don’t really see a need to get to 100% adoption or whatever. If you have a simple static-site blog that updates every month or so I don’t think it is a big deal that it doesn’t support WebSub.