I could add just one clarification. The reason was that they did not issue an invitation to the OSCE in a timely fashion, by the deadline that the OSCE's ODIHR arm needed to organize a proper electoral mission. At the last minute there was a move from Belarus, but it was far too late for the OSCE to be able to mount a credible mission.
On November 5th, 2020. See this statement in context.