I expect the full postmortem will dive into this in more detail, but one reason mentioned in ekr's post is that many add-ons are distributed independently, outs

I expect the full postmortem will dive into this in more detail, but one reason mentioned in ekr's post is that many add-ons are distributed independently, outside of AMO. So while we could re-sign those, we're not in a place to re-distribute them. Instead, "users would have to manually update any add-ons that they had installed from other sources, which would be very inconvenient."