I was all for the 5X because I loved the original nexus 5 to death, and I figured the 5X would be the same as the 6P with just a slightly lesser processor and different form factor. I also assumed the 6P would be priced so much higher I wouldn't even want to consider it.
But then it turned out to have a different type of screen (lcd vs amoled, which wasn't the best ever in the n5, a little washed out, and I saw journalists say the same at the event), no stereo speakers, worse front facing camera, lower screen-to-body ratio (than both the 6P and the original N5), thicker with a smaller battery (than the 6P), 2GB ram, etc. Suddenly, while I knew it would still be a great phone and I'd be 'happy' with it, it felt like as a $429 investment, it wasn't as big of an upgrade to the original N5 as I thought. For $70 more I could get a phone that was a pretty significant upgrade in every way and much more future proof. Hence, shifted interest to 6P