1. Well right now there is no "why" because we have seen few reviews to go on and there's no history to see who hates and who likes what they bought.
2. Right now all we are seeing i the 1st stepping boards ... as bugs are found and corrected, later steppings will have modifications that eliminate any of these and tweak performance a bit.
3. Feature wise
https://www.newegg.com/Product/Productcompare.aspx?CompareItemList=280%7C13%2D130%2D973%5E13%2D130%2D973%2C13%2D132%2D930%5E13%2D132%2D930%2C13%2D132%2D928%5E13%2D132%2D928%2C13%2D128%2D966%5E13%2D128%2D966
All support RAM speeds higher than you'd likely venture to spend for (3866)
All have same audio solution
Giga has a better LAN solution than other 3
MSI has 6 USB 3.1 ports, others have 4, Giga adds Thunderbolt
MSI has 6, Giga has 5, Asus has 6 4-pin fan connectors ..Asus adds a 5 -pin connector
Asus has 2 RGB connectors, others have 1
I see that Giga has followed MSs lead in adding full steel EMI / reinforcement to the PCI slots which I like a lot. Asus has done so also but it doesn't look as extensive as the other two. We aren't doing Z270 builds (See Item 2 - prolly start in may) so haven't had our hands on any.
Out of last years modes (Z170) The Giga Gaming 7 was my fav board under $200. The Z270 version again seems to be the most popular and well reviewed board so far in the < $200 category, but the looks are not for everyone.
If ya pushed me to make a choice based solely on what we have seen so far, and having no idea what features, aesthetics etc are important to you, I'd take the Gigabyte K7 and if that's not in the budget, the Giga k5
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