Take a look at CoD:MW2 sales figure across platforms. From Infinity Ward:
"Yes, PC is the smallest percentage in terms of how much sold on each platform but that hardly means anything other than the PC is just the smallest market"
By smallest percentage they mean a measly
3% of the first 1.7 million copies sold in the UK.
Now realise Steam represents a subset of that 3% PC market, saying that hardcore PC gaming is "popular" looks pretty darn silly. Valve and Blizzard, the two biggest PC devs, make engines specifically made to run on a range of machines for a reason.