I do think the "no one ever got fired for picking Microsoft" is part of it, but it's also a very well developed eco-system for a mid-sized shop that don't have the man power or expertise. With MS you have tools like group policy and tons of vendors do tie ins with Active Directory to make offering single sign in services (VPN, email, extranet, etc) very easy.

People are down on MS because they feel that it's just the "easy" decision, but they also have a mature and well developed product that works pretty well for a midsized business with a small IT staff.