that's because MS killed it. There were more than a few games that became ps2 exclusives due to MS killing the xbox.
Well in order to be sold "at cost" or with only a slightly part of the price being subsidized MS would've had to get the rights for at least the GPU. Moving to an AMD CPU would've helped keeping costs low, but if they were able to secure the rights for the CPU too (be from intel, AMD or even Transmeta, which was making some good X86-compatible chips at the time) a $100 MSRP with little or no loss would've been possible at the end of 2003 or during 2004 (like the PStwo) and in mid 2005 they could've released a more cost-effective version using a SOC array. Anyways, I couldn't find more info on that Xbox prototype that uses an Athlon, anyone has a link to said article/news?