Internet propaganda mostly.
Now lets get this clear, the D750 is a great camera, there is really nothing wrong with it at all. The problem is it is placed between 3 other camera bodies and is not well differentiated.
I have mentioned the following points before but coincidentally Thom Hogan has just released an excellent review of the D750 which highlights exactly my issues:
http://www.dslrbodies.com/cameras/current-nikon-dslr-reviews/nikon-d750-review.html
The D810 is clearly a much better camera, everything is either identical or better, including the sensor and not just more pixels, lower noise, higher dynamic range, better detail retention, better colors. Also no AA filter, the resolution difference with the extra sensors and no filter is really quite significant. The D810 is much more pro-orientated. then there are things like the buffer is far bigger:
https://photographylife.com/nikon-d750-buffer-capacity
D750 (12 bit) : 25 photos and filling in 3.8 seconds,
D810 (12 bit): 47 photos and filling in 9.4 seconds.
For weddings, the D810 has a noticeably quieter shutter than the D750. In fact, the D810 in it is regular shooting mode is quieter than the D750 in its quite mode! The D810 also has a faster flash sync speed,
With the D810 there is a clear price premium as there often is at the high end. However, it should be noted that the D810 is really like 2 camera bodies in 1. The 16MP DX crops at 7FPS is incredibly useful and at the moment is about the fastest Nikon DX camera that has suitable resolution. So a D810 is like owning a D750 + D7100 and more all in 1 single body.
At the other end of the spectrum the D610 is incredibly close to the D750. They are more similar than different. If you are happy with the D750 it is almost certain you will be happy with the D610, and the D610 is incredibly cheap now. You get a very similar body, identical senor, very similar AF system that covers a similar area, same prosumer orientated features.
Really, you either have the budget for the D810 or you want a low cost FF solution. The D610 is £500 cheaper than the D750 (and is often found much cheaper refurbished), that is enough to by one of the nice new prime: 20mm f/1.8, 28mm f/1.8, 35mm f/1.8 or 50+85mmf/1.8 and with plenty of change on some of those for bags/batteries/flash.
If you shoot wildlife, sport, landscape, studio, architecture etc you will want the D810. You pay the premium but get the best and have the flexibility of a superb DX camera when needed. If you primarily shoot weddings/portraits the D810 is still superior but either the D610 or D750 will suffice and will be more or less indistinguishable.
Thus I just don't see a place for a D750. Sure if someone offered me a D750 or a D610 for free and I can pick just one, I will pick the D750 every time. If I did end up with a D750 I would still be buying a D7100. D810 is just a plain cheaper setup for nature togs.
If I didn't ave the budget for the D810 then i would be looking at refurbished D610/D600.
The D750 is sandwiched between tough competitors and I feel you either want to go one way or the other.
And that is not forgetting that there are plenty of other choices:
Df: D4 sensor on the cheap, retro, small, costs only slightly more than the D750
D800: Slightly slower than a D810, so if you don't shoot sports it is a bargain and the same price as a D750.
D600: Near incidental to a D610/D750. Bad reputation because of Nikon's monumental PR disaster wrt oil spots, but Nikon will change the mirror for free. Can be an utter bargain.
D700: Older but faster than any of these if 12MP is sufficient. Very cheap.
D3/D3s: can be had in similar price points second hand.
