Actually, this isn't the only issue, as Tale pointed out, the lines are correlating, but that doesn't mean that they're directly affecting each other.
Fact is that the games generating the greatest sales are those with an action element. Slower (esspesally turn based) games are, frankly, being phased out due to a market that isn't interested in them, and that includes PC exclusive titles.
Take a look at Homeworld 2, they followed the trend set by the later installments of Blizards RTSs and opted for a fast micro-heavy RTS. This was a COMPLETE turnaround from the original and yet, not a console in sight.
Look at Nexus, the 'perfect' PC game and yet the lack of sales forced the company to shut down.
Again, look at CnC3, they took all the features that made the original great, but then put the speed up by about 300%. Sure, it was released on consoles, but this was really an afterthought and it's very hard to pin any of it's 'new features' on 'simplifying for consoles' (esspesally considering that RA2 did exactly the same thing).
Consoles are a much larger portion of the pie than they used to be when games were, in general, slower but the 'problem' is much wider than that and can't wholly be blamed on games being cut down for consoles. I'm not even convinced that CoD2 would have been different if it had never been released for consoles.