I think part of it is also that games got so good. If I look at my 'only' four installed racing titles, each does exactly give me the right thrill. There's so much content and long-term motivation, coupled with awesome gfx - why would I buy something new?
Same likely applies to flightsims. The few we have today are all awesome. And ultimatly the speed of technical progress has slowed down. There's no new 'killer feature' round the corner that can sell a sim. CPU speeds and GPUs are not evolving as fast as they used too. The featureset jump from CFS2 to IL-2 was huge. The jump from IL-2 to CoD much less so (yes, more details in everything, but groundbreaking new innovations?). Each new generation can only offer incremential change. The next big improvements will come trough pheripherals (3d, head mounted displays) just as TIR did cause a massive jump in realism.
Which leaves us with non-sim games offering an interesting interactive story and some action. But they get shorter each iteration.
