I would say the issues with pretty much all your examples save 1 or 2 is because the latest iterations are TERRIBLE compared to their prequels.
Developers have either gotten lazy or retarded and I think they fact that they changed their games so much from what their fanbase wanted is why most of the ones up there on your list show big declines.
The only other thing to consider is that used games might have been much more popular this generation with rising retail prices over last generation, as well as an increase in piracy over last generation perhaps? Or quite simply, fans of those older series grew out of gaming a bit? I mean seriously, but the time you get to over 3-4 games in a series, well over 10 years could have passed and peoples lives change a lot in 10 years.
In the last 2 years, I personally have seen a major decline in my gaming time. I began working a full time career after graduating college, I got engaged and soon to be married, I bought a house which involves more chores than an apartment (lawn care/landscaping especially), I took a greater interest in my physical fitness since my metabolism finally started to slow down a tad in my mid-twenties. Even just a year ago I was buying probably 1-2 games per month, where as now I am buying 1 game every 3-4 months.
