Why aren't my motherboard and gpu compatible?

Nersheti

Prominent
Jun 8, 2017
1
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510
I'm trying to build a gaming pc but I keep having problems. A friend helped me pick out my components and assemble the computer, but we couldn't get the gpu to work. After taking it to a professional, he determined that the graphics card was bad, so I got a replacement from newegg that arrived today. It still won't work. After contacting evga and asus, I've been informed that my motherboard isn't compatible with my graphics card.

Specs:
Mobo: asus rog strix z270e
CPU: i7 7700k kaby lake 4.2
Gpu: evga geforce gtx 1080 ti
Ram 16gb (2x8gb) ddr4 2400
Psu: 850w

Questions
1) why wouldn't these components be compatible? The research I've done suggests that they should be.

2) assuming that I'm forced to replace the board (I'm definitely keeping the gpu) I'm thinking of getting the ROG Maximus IX Formula. It's probably more than I need, but I like the option to add liquid cooling easily if I upgrade later. I've checked and it is compatible with the 1080 ti. Is this a good choice, or should I go with something else?

3) since I'll be keeping the CPU, can I use the ssd I'm using on the current build that I've already installed windows and other applications on, or will I need to reinstall everything on a fresh drive? Both mobos are z270.

4) to avoid this in the future, what specs should I pay more attention to when selecting mobos and/or gpus? I thought I had been thorough, and everything else works fine, but this is a pretty major hiccup.