> Windows was the OS that the company was also using for employee machines, so they knew it.
They understand it, have an idea about the cost and amount of support necessary. "They knew it" seems to convey that they were essentially stupid or not adventurous enough to try something else.
> The IT department only wanted Windows machines on their network
Again: a complete valid reason in my view. Here on this forum you have even people advocating that server and client in web apps should be written in the same language to keep things consistent.
> The customer was afraid of Mac OS X because they thought it would not be stable enough
You make that sound like there teeth were rattling in horror, but apparently failed to convince them that OSX would be stable enough?
I'm not a Windows developer or -user, indications that win 98 runs on my ATM also make me concerned.
But "muhaha, they are just stupid bricks for using windows" (no, you did not say that, but it sums the attitude) shows a lack of understanding and, as a consequence, does not advance the spread of alternatives into these environments IMO.