The word "canonical" limited what Star Wars: The Force Unleashed could aspire to be. In abiding by the established Star Wars fiction, The Force Unleashed's designers had to play by rules similar to those of a time traveler, treading carefully around certain events for fear of upsetting the delicate balance between what's already happened and what they want to add. Starkiller's journey was a side story that couldn't directly touch the tale of the movies - just set them up. Only the "evil" ending, where Starkiller offs Darth Vader and replaces him as the Emperor's servant, diverted from the sanctioned mythology. This ending, of course, was not canon.
When fandom can be split into tiers of knowledge, the most die-hard fans feel like their investment in a franchise was worth the time it took to become experts on a series.
As the importance of storytelling in games increases, developers are taking steps to establish games not as their own individual tales, but as pieces of a larger franchise's story. In launching a game alongside a novel, comic book, internal wiki, and an announcement of a trilogy, developers are building volumes of lore before the first game is even out the door. Little about a franchise is now left up to chance, and every piece of meaningful content must fit into an overall story. This has its clear advantages; the Mass Effect trilogy, for example, thrives on its pages of codex entries, prequel novels, and supplementary games.
These kinds of auxiliary fiction pay off in allegiance; when fandom can be split into tiers of knowledge, the most die-hard fans feel like their investment in a franchise was worth the time it took to become experts on a series. Simply put, it's cool to know more than other people, especially if you can show that knowledge off and inform someone at the same time. For creators, having a compendium of knowledge to work from means not having to create concepts and characters on the fly - they can see what's already there and work off concepts, characters, and locations established elsewhere. The most obvious example of this, Halo: Reach, took a novel that was supposed to be backstory for the core series and made it the forefront of an entirely new game.
Metal Gear is another prime example of a franchise where previous knowledge informs appreciation. Metal Gear Solid 3 may be the earliest entry on the series' timeline, but even in telling a story distanced from the rest of the games, it's best appreciated with a fan's eye. A nod here, a young Revolver Ocelot there, MGS3 is teeming with references to the rest of the franchise, and one of its biggest payoffs is witnessing the birth of several story threads that won't necessarily resonate with someone who doesn't follow the series. Anyone approaching the game as a newcomer can relate to themes of MGS3 as a singular work, but only those who've played any other entry will appreciate what MGS3 does for the larger Metal Gear story. And it's this appreciation of a story arc told over several games that can create superfans waiting on any morsel of info fed to them; ask Half-Life fans what Portal 2 means for both series, and you'll see what I mean.
Unfortunately, Metal Gear also has a great example of how fiction can burden a franchise. Metal Gear Solid 4 sits on the opposite end of the same timeline as MGS3, and as a result is heavily weighed down by its canon. For every time MGS4's narrative moved forward, it made a long-winded allusion to the past. Every time it brought up a concept from a previous game, it had to bring the audience up to speed on why it mattered to the current plot. Tasked with bringing closure to the various loose ends of a story begun in an era when plot was an afterthought, MGS4 could only succeed by telling rather than showing. At the very least, Kojima Productions wasn't confident enough that someone just getting into the series could jump in without being briefed on every bit of minutiae.