Others, who ripped the DVDs and uploaded the clips. Most of the HD good quality ones are gone, definitely happened recently. Yeah, they're breaching copyright, but meh.
On the topic, NBC blocked the YouTube upload of U2's awesome performance of Elevation on SNL (even An Tha would like it). And they don't have it available on the NBC website, so that one is gone to the ether and will never be seen again.