As far as why they weren't released earlier to the American public... my understanding is there were security concerns, specifically that they showed interior views of restricted areas of the Capitol.. the "escape" routes that people used when fleeing the protesters. I know there are tunnels that lead out of the Capitol.
McCarthy promised to release the videos as part of his "deals" he made to win the Speakership. He chose to release them only to Carlson. After McCarthy was removed, the new Speaker promised to release them to everyone.. again, part of his "deals" to get the votes to become Speaker.