Half-Life 2's episodic content provided tantalizing nuggets of story while their brief four to six hour length made them completable on a lazy Sunday afternoon. This would have worked brilliantly had the episodes actually stuck to a short release calendar, but with 16 months between Episodes One and Two and Episode Three MIA for the past three years, it seems that Valve bit off more than they could chew. This was not the case with Telltale's bevy of episodic adventures. Sam & Max, Tales of Monkey Island, Strong Bad, and more recently Back to the Future have pleasingly held up their end of the bargain, each month releasing a two to five hour episode of the various series on time.
Long-format storytelling is also difficult in games is due to hardware incompatibilities. To play Shenmue (legally), you'll have to dig up a Dreamcast and order the disc online since very few retail stores carry games older than a few years at most. De Blob 2 is available on PlayStation 3, Xbox 360, and Wii (as well as a separate DS version), yet its predecessor was a Wii exclusive. Those who own a Wii might be able to track down a copy of de Blob, but those with only a PlayStation 3 or Xbox 360 may not be so eager to jump into a game with a "2" in the title if they haven't played its predecessor.
This was also the case with the convoluted Kingdom Hearts series. The first game was a PlayStation 2 exclusive, but it had sequels on the GameBoy Advance, Nintendo DS, and PSP. For a series steeped in heavy mythos, that's expecting fans to pick up a lot of hardware just to completely keep up with Sora's adventures in the Magic Kingdom.
Even if we have the necessary hardware and spare time required to start an ongoing series from the beginning, games are a medium reliant on ever evolving technology, making it harder for them to withstand the test of time. Graphics, control methods, and even game design are constantly changing and what was the norm a decade ago often feels archaic when viewed under the harsh lens of the current time.
When I tried to play Metal Gear Solid 3 I found it completely impenetrable, having not played the first two. What I found was a game that controlled terribly, had awful voice acting, and didn't look as good as what I'd grown accustomed to. This was all par for the course at the time and it was still a great game, but playing two older games before I could fully appreciate one new title was a herculean effort that many would simply not bother with.