For a new IP at the time, and extreme hardware requirements ...
Great games make free marketing.
A thought:
You see .... it's about that PC simply became a much higher standard platform, more mature and professional.
The problem is, that all those rookie inexperienced developers who have no good experience on PC, always whine about excuses.
Publishers are run by economists - lazy spoiled profit school a lot of people go because it's easy, unfortunately they don't teach how to manage a good gaming company there.
Btw Starcraft 2 sold about ~5 million on PC (only PC) .. and growing.
PC games sale over time, not in one single blast, people don't sit on PC every day looking for games to come out, they notice it later, and it takes time when they decide to buy some time for it.
While Consoles constantly have people playing
When i was that age, consoles were exploding too, those friends that didn't had a console (only PC) used to play it, so it spready like wildfire in the teen population, do you see any 30y olds calling all their high and elementary school guys to play super smash bros together every week.
It's so simple, the older you are you entitle your self to you're own decision, and so you decide what and when you're gonna do, when you're young you have ofcourse nothing else to do but that's not a bad thing, it's just normal that you jump on something right away what a friend recommends. but PC gamers became hesitant, because it's obviousl a mature
Some people went on to stop playing games at all, for like 2 years or more, they might buy 2 games or only 1 per year, and that game has to be exceptional or legendary to attract them.
Most of my friends i know , they used to be PC pros in the age of 10years, i followed soon, most of em still play probably, but much less visibly, much more on their own will.
The older you are, you don't expose your self as a gamer that much in public - ofcourse many are hiding the inner geek, but that's not a bad thing.
This is more of a mature demographic, but there are others as well, only a great game catches them all at the right time.
The industry however relies on statistical (artificial) data which paints only a proft-maximization view and not the background of what, it's not answering WHY this why that.
The majority industry just sees customers as an avreage american joe, and completely fails to recoginze the true market. Mostly because many people don't care about career and critically acclaim, more about money, shame.