A number of reasons. I should first mention that my primary supporting reason to get a good camera again was that our band has things we need one for, and that I have bandmates who are not interested in photography that use it here and there. While I do love photography, I would not have been able to justify the expense (and boy are cameras expensive!) if it didn't serve the band.
Better AF - I never use AF, but my bandmates need it. Which is why I have an AF zoom that I never touch personally. The D750 AF is especially better in low light, where live shows and band things tend to happen a lot.
Control - I started on a D70, so the traditional control scheme is anything but familiar to me.
Video - We will be using it for music videos. Which I should be figuring video out for, but rarely even remember it does video to be honest.
ISO/DR - While the Df technically has less noise, I find them to be equal when down-sampling is brought into the picture. Combined with the ability to lift the shadows more on the D750, the D750 can be pushed an extra stop in low light venues without falling behind in IQ. An extra stop means a lot between iso 8000 and 16000.
Grip - It's not uncommon for me to be holding my camera when a crowd of sweaty people suddenly slams into me, so having a deep and secure grip really feels nice. Frankly, I'd be scared of the Df flying out of my hands sometimes.
My only experience with a traditional camera design was a Pentax MX which was really nice to hold, and I would switch to such a design quickly if it's capabilities met my needs and the viewfinder was that good. Sadly, no DSLRs is, and I don't see that happening with AF being the standard.