Cause it's the best quantitive measure out right now, you can't compare sales in absolute terms but in relative terms. So if one dude sells 5k it's safe to say that someone selling 50k is hotter. It's also about how much you push your album, how your fan base is split up like I can imagine that country artists have more fans that actually go to stores and buy CDs, pop & hiphop might have more iTunes fans, hiphop & electronic dance music more illegal bootleg fans, classic rock more fans that like your old shyt and go to your arena tours for the nostalgia etc.
So it's still a good measurement to see like Wale is obviously hotter or has a much more solid fan base than say B.O.B..