SGI is standing down Irix and MIPS architectures. Likely one of the harder decisions they have had to make, these would not have been selling much as of late. MIPS was hopelessly long in the tooth, and Irix, while one of the better Unixen out there (IMO), was closely tied to MIPS.
In the end Irix could not keep and continue to attract applications. When this happens enough, your platform becomes less desireable. Similar things are happening to other *nix today.