It's simple: because they can pirate proprietary software and they always perceive something which has a price tag as more valuable than something which is offered for free. Something which has more (perceived) value but which you can obtain for free will always win against something which is offered for free (and thus is perceived as probably worthless).
Sad but true.
There are some exceptions, though. A lot of people use Vlc, for instance, because it can play every single (probably pirated) video you'll throw at it. And there isn't a market for commercial video players. Something like Firefox wouldn't have reached such penetration without a lot of marketing (most people doesn't even know what a browser is, go figure).
I'm constantly baffled by the number of articles about these subjects coming out every day from authors who don't understand (or completely ignore) basic human psychology.