John Gruber on Daring Fireball adds his views on why people continue to choose PCs over Macs (emphasis is mine):
Assemble a group of intelligent and curious people who are mostly inexperienced with computers. Sit each of them in front of comparable Mac and Wintel boxes and give them a few hours to explore. I’m confident most of them will prefer the Mac. The Mac OS is easier to explore, more intuitive, more consistent. But this isn’t how most people buy computers. They don’t take a day or even a few hours to try different systems. Fear of blowing $1000 or $2000 on a “bad” computer motivates them to buy whatever is recommended by their closest nerd.
Choose, not prefer:
And so the question isn’t why don’t more people choose Macs, but rather, why don’t more computer nerds choose them?
There is one more reason, which is rarely seen because it is rather controversial, but since I've heard it so many times not pointing it would be hypocritical. It's software piracy. In addition to "what would you recommend?", people often ask for software, and they don't mean to buy. So the nerd willing to "share" software will close the deal on his/her platform of choice. The perception that there is more software on the PC platform may help, but the question is more about the easiness of piracy. Not so long ago, pretty much all versions of Windows would happily accept 111-11111... as a valid license number. If that was not an incentive, what was it?
In the same article, he also has an interesting point of view about Outlook:
Outlook and Exchange are very good to one class of people: IT. [...]
An Outlook/Exchange installation at a medium or large company requires a decent chunk of IT manpower just to stay afloat. There is no good reason an email system should require so much effort to maintain. You can argue that Exchange and Outlook offer much more than just email. But that’s not an excuse — it’s part of the problem.
This goes along the lines of the Anti-Apple IT conspiracy.