The risk of a failed cell in a 6 volt setup is overblown.
A single 12 volt battery has 6 cells. Two 6 volts have 6 cells.. So all things being equal, a single cell going bad would be the same failure rate with either... But all thing are not equal. The 6 volts have a much more robust construction and will most always far outlive a 12 volt
12 volt fans tend to worry about cell failure as they are used to it happening on their 12 volt batteries.
6 volt fans realize the the odds of a cell going bad in a 6 volt setup is much lower... So they don't worry about it.