Past history and reputation. Gigabyte and Asus have proven themselves to be reliable brands and a safe recommendation. Zotac hasn't quite got the same reputation for quality and reliability - which is not to say it's any worse, just that they haven't earned the same level of respect among enthusiasts yet.
I've had Graphics cards from many different brands in the past. When budget was my first consideration I'd buy the cheapest version of a particular GPU I could find and it generally worked out ok, I've never had a graphics card fail on me in the warranty period regardless of brand and in fact the only cards that have died on me have been more than 3 years old.
That said I tend to stick to Asus and Gigabyte these days because they are...