So, after ~2 years, my colony has had exactly 2 raids. The "tutorial" naked brawler, and a basic 3 man pirate raid. I'm playing on a temperate forest, so it's not like weather conditions should be affecting spawn rates. My colony is also on Phoebe challenge, but 2 raids seems a little excessively easy. Other events have been failing to trigger as-well. I have not seen any (orbital) traders, Manhunters, Crashed ships, or infestations despite being able to receive all of them.
[EDIT] I also haven't had any social fights, relationships. or extreme breaks happen
This issue doesn't seem to be restricted to my game either, as it seems other people are experiencing the same issues.
https://www.reddit.com/r/RimWorld/comments/4tbifm/no_crashed_ships_manhunter_packs_and_infestations/
Fixed in A12b :)