Because the documentary started filming when only Alpha was out and follows the story of Notch, the game and Mojang up until the first Minecon and the official release of the game. So they use lots of footage from earlier versions.
Edit: Also the opening bit is actually a fairly well known Youtube video released during the early days of Minecraft. In fact it's probably the video that got me interested in the game. They use a lot of Youtube videos in the documentary.