I'll join in on the why Intel/Nvidia instead of AMD/ATI
First, AMD owns ATI, or at least used to, haven't kept up that close.
We all know that a program written for windows 2000 kinda can work on a later windows machine, but not that great, if at all. This is due to the programming interface that is available to progrsmmers to tell the machine what to do. Programmers (most of them) don't write code that directly tells the CPU or GPU(Graphics Processor/Processing Unit) what to do. The vast majority of programs are written to take advantage of all the 'simplified' programming languages, called high level languages, far away from writing thigns like the following where each line does some very specific thing to an absolute specific address of memory:
mv 345238, 498730
jmp 43982
poke 4573, 83476
that mess is code like would be written by pure geniuses writing for 'bare metal' directly to/on the processors.
most modern code looks more like this in its human readable form, before distribution as a piece of software, a specialized program called a compiler turns this below into something mere normals cannot comprehend that is little more than an incomprehensable string of hexadecimal pairs.
void CSSdecrypttitlekey(unsigned char *tkey,unsigned char *dkey)
{
int i;
unsigned char im1[6];
unsigned char im2[6]={0x51,0x67,0x67,0xc5,0xe0,0x00};
for(i=0;i<6;i++)
im1=dkey;
CSStitlekey1(im1,im2);
CSStitlekey2(tkey,im1);
}
Why in blue blazes does any of that matter?
Nvida and ATI use mildly different programming interfaces. They design the hardware differently, want larger marketshare, and different optimizations for different things. There are 'agnostic' programming languages, but that lays a whole new problem of being once again still further from the 'bare metal' and having less control over the full output capabilities of your hardware.
Really short version. Instead of having minimal control of what show on the screen, Exocads and 3Sheep decided to optimize their code for Nvidia hardware instead of ATI hwrdware.
For the why's and wherefores of that decision, one or both of us need to be drunk.
(Edit: I am a mere normal, anyone who actually can write decent code is rolling on the floor coughing up a lung in laughter)