Because the Gigabyte is $399 where as the MSI 670 Power edition is $429. If you were settling on getting a 670 why would you pay 30 more just because of the brand. Both Gigabyte and Msi are known to be good companies.
A similar type model for 680 is around $550 or a so maybe less.
And if you are forking over $300 + for a card why isn't it important to save what you can if you are considering high end models on both ends.