Why? Largely because there's no option, if you're in the Canon system and need a truly rugged camera then the 1DX line is your option.
Remember how they used to split the 1D and 1Ds lines? That was a great way to leverage R&D and production cost whilst catering to the needs of photographers. Sadly the 'merged' 1DX was a step back in many ways.
Not really. The people who wanted the higher resolution rarely needed the durability of the 1D, and Canon put out the 5DS R (which itself is hardly flimsy) for that crowd. As much as the 1Ds mk II will always have a special place in my heart, ultimately they were all work tools and when they effectively scrapped the 1Ds line and moved that function over to the 5D body, it was quite a relief. Having the 1D X set-up ready for action and the 5DSR sat in the studio tethered was a perfect working pairing.
In my opinion this camera is not for someone currently shooting with a 1DX 1DXMKII or 1DXMKIII
Canon literally said this was not a 1D replacement. They have always, from the first second they publicly acknowledged the development of this camera, been very explicit about what it was intended for (sports, i.e. low resolution) and where it would fit in the product line (i.e. below the 1D and eventual R1). They've even said they called it the R
3 specifically because it was not worthy of the '1' designation and that they didn't feel RF tech was far enough along for anything to be called '1' yet.
In any case, 24mp is high for a sports camera.
I know several full-time sports pros in this area who use the 1D X mark III exclusively in the medium jpg setting (12.7mp) because even 'just' 20mp jpgs is overkill and pointlessly inconvenient for fast turnaround. Moving up to native 24mp isn't going to help them; at a certain point oyu're just paying for parts you're not actually using and adding on to the CPU's processing load for no benefit. I'm moving to sports full-time soon myself and I was not anticipating using anything above the 24mp mark myself; I always like 24mp for the neat 6000x4000 which satisfies my brain's fixation on having everything rounded off to clean values, but realistically I know that it might be too much at times and I was
dreading the rumoured 30mp.
Sometimes I
do want higher pixel density. For wildlife I've already got the 7D2 (I don't hold out much hope an R7 will actually be up to the task, but we'll meet that when we come to it) and for anything closer or moving slower I've got the R5 and R. The R3 was never going to be taking the place of any of those, it was never designed to be as per Canon's own statements, and I was never hoping it would. They said it'd be a sports body, I wanted it to be a sports body, and sure enough it's turning out to be a sports body. Brilliant.
If someone wants very high resolution (I'm loathe to say "need" because approximately 0.00001% of shooters actually ever
need such high resolution), the R5 is right there and cheaper than this R3 will be. The 5DS R is already there, too, for even more detail than the R5, at least at low to medium ISOs. There's also the Nikon D850, the Sony a7R line, and the Fuji GFX cameras, all ready and waiting. If someone is the kind of person who insists on always buying the most expensive camera no matter what then they can go drop their platinum cards on the 150mp Phase One.
Not every product is designed to meet every requirement or desire. Let the stills cameras be stills cameras, the video cameras be video cameras, the studio cameras be studio cameras, and the sports cameras be sports cameras. I am looking forward to the R3 more now that it seems they are indeed keeping it as optimised for sports as they first promised.