Hulu is awesome for users, but not so awesome for content producers, as it doesn't make all that much money. It certainly doesn't make enough money to pay for all those shows being made. The solution? A subscription service that gives you access to all the old shows.
It appears that Hulu is moving towards adding a $10/month subscription service that lets you watch all the shows from a series rather than just the most recent handful. Those will remain available for free. But as Hulu is moving towards other screens, such as TVs and iPads, it's understandable that they've gotta try to make some cash from all these viewers.
Via LA Times