That's because, up until recently, today's users only had a choice of buying a Windows-based PC or buying a Mac. Microsoft knew all along that people are resistant to change, so did everything they could (including some very dodgy business practices) to prevent hardware manufacturers from offering alternative OSes. Get people to learn using a PC the Microsoft way and they won't be keen on having to unlearn things to do it the OS X way or the Linux way or the "any other" way.Heh. Indeed.
Linux is very good but still requires too much research to get it working at a level that today's users are accustomed to.
They still use this tactic by trying by whatever means necessary to get Microsoft products used in schools, or in 3rd world countries, etc... so that their user base isn't aware of alternatives that could be used instead.
Users that have learnt to use an OS other than Windows find that Windows is bizarre and user-unfriendly - there's a famous quote by an American astronaut asking if he really had to click the "start" button in order to shutdown his PC

It already is changing. Dell now sells desktop PCs with Ubuntu pre-installed. Netbooks with Linux installed are available on the High Street and at virtually all online sellers - including OcUK. In June 08, 3% of PCs sold in the UK had Linux and not Windows installed on them, according to a report by research firm Context.Will this ever change? No.
I disagree - there are plenty of companies using Linux, such as IBM, Google, Intel, Boeing, Cisco, Nasa, Virgin, Peugeot, Disney, Shell, Panasonic, Ferrari, the US Navy, Amazon, Lockheed Martin, Mercedes Benz, Deutsche Bank, Volvo, etc... All of these companies use Linux. I'm fairly sure they use Windows too, but they all definitely use Linux either on servers, workstations or both.There's not enough money, and people, involved for hardware and software companies to jump on the Linux bus with both feet.
The 'community', while having many helpful people is in general self destructive. Bickering over distributions, how to do things, who knows best etc.
Conflicting ideologies and advice are often as useless as no help at all.