Because a global pin is something you want to make absolutely certain everyone is aware about, no?
If it doesn’t show up in the digest, then we’re not doing all we can to make sure everyone is aware of it.
The equivalent UX in the digest email that is comparable to the web, would be to keep sending it at the top of the digest until either (a) the topic is un-pinned, or (b) the user dismisses the pin, whichever comes first.