re: Does Microsoft Know Best?
And you could add Dell with their KACE product - while it'll work well with desktop/laptop/server hardware from any manufacturer, you get a few more bells and whistles when using it in conjunction with Dell hardware.
I think a lot of this has to do with the mindset that generated the saying "Nobody ever got fired for recommending IBM" - having an infrastructure loaded with components from one manufacturer so that the management of that infrastructure is easier is a safe play. The lower the amount of time spent on administering the environment, the lower the headcount in the IT department can be and still get the job done.
Same goes for security solutions - a number of organizations that I'm familiar with use one vendor's products to protect every possible attack vector - anti-virus, anti-malware, client firewall, anti-spam, etc. While this may make administration easier (one console to run everything), you also end up with more vulnerabilities when that one vendor's products are compromised.