Who was there (or popular) first has a lot to do with sales too.
The iPod was the first very successful mp3 player. It's popularity is part of the reason, I went for a iPod Mini as my first mp3 player. I got used to iTunes, to the click wheel, bought a car adapter and the product satisfied my music listening needs.
Since then plenty of mp3 players have been released and I have no doubt some of them are of superior quality to the iPod but each new mp3 player I bought over the years have been iPods because that's what I know, I feel comfortable with the interface and I'm still satisfied with what they offer me.
The same logic applies to those (mindnumbingly boring) racing sims. Gran Turismo was there long before Forza, people know it, people are satisfied with the experience and that's why the game performs better.
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