Looking at the recent rise of Knoppix and the spate of live cd distros following it, I have to comment on the single main reason why this is so. I know there are alot of *little* reasons, but surely the most critical one is not needing to re-partition your HD.

Hard Disk Re-partitioning is a black art, as I discovered recently when I heard an expert say that the famous Powerquest "Partition Magic" actually does not *really* re-partition, but only fools the system into believing that is so.

Consequently risk-averse users with windows already on their HD couldn't face dealing with partitioning and now with Knoppix they don't need to. And that's one big user base.