Internal wiki hits the same problem as email (although partly because companies always seem to choose terrible wiki implementations (confluence)). Too much information goes on there, terribly organized, and no-one ends up being able to find anything. Mostly these kind of things are immediate announcements that need to be acted on once and never again, so a semi-permanent store isn't appropriate; also the whole point was that everyone needs to be notified, but people can't watch every wiki change.

Blog I haven't seen tried, but I imagine it would hit the same problems; either you notify everyone and have the same problems as slack, or you don't and have a big risk of someone missing an update.