It's an interesting question. Telltale's own Bone died before its time, SiN did likewise, and most full game sequels come around faster than Half-Life 2's "episodes". So why, the, does Sam & Max stand as the only successful episodic series to date?
For the other companies, Valve and Ritual, they made a lot of the mistakes we did early on. But we stuck to our guns and kept going, while they had so many other things going on at the same time. They bailed out a little earlier. We were all in it together then they dropped off.
That was Telltale's Dan Connors basically saying that since all episodic gaming's going to make mistakes, best to make sure your episodes are as cheap and fast to produce as Sam & Max is. Budding episodic developers, take note.
Q&A: Telltale tells why Sam & Max works [GameSpot]