Brouhaha Abounding has a point when it brings up Empire Strikes Back. Along with The Godfather Part 2 it's often cited as one of the few examples where a sequel is considered better than the original. In both cases there's a darker more hopeless tone (Though The Godfather was never happiness and light to begin with of course), it's not much of a surprise that developers, most of whom are of the Star Wars generation, would look to those works for a references of how to handle a sequel.
Of course the same is true in a lot of cases, the Harry Potter novels progressively got more mature in tone, with more death as the series progressed. This made sense given the maturation of the central characters but combine it with the Empire Strikes Back syndrome and it does serve to create a perception that a sequels need to be more mature to be successful.
The culture and climate of the time is another factor, over the course of the twentieth century entertainment seems to have gone through cycles of optimism and pessimism, and considering the prevailing political and cultural climate of the last eight years it’s hardly a surprise that mainstream entertainment has seen an increase in battered, wounded and darker protagonists.
Writing for games is still not as good as it could be; I don’t just mean dialogue but also general plotting, structure and character development. There’s still a heavy bias towards blunt easy to recognise visuals over nuance or subtly so it’s no surprise the more obviously mature aspects of games are visual ones.
