Its the fact its an all in one system that is relatively easy to use for the basic user. iMacs for instance are the entire PC minus the keyboard and mouse, they dont take up a lot of space and are esthetically pleasing (as are most of the mac pcs). People will buy macs if they dont have a lot of computer experience and want something that is supposed to give them less hassle then a pc would. I would get my mom a mac for the sole purpose of not having to have to deal with emails or phone calls on 'how to do this' or 'the pc is telling/warning us this'. I on the other hand wouldnt buy one for myself or those who know/have indepth knowledge of computers for the fact that its not as useful for gaming, for programming (most business run programs that are on the microsoft .net platform and the fact its easier to access in the C language family hardware then it is in macs), as well as the business would and well engineering.
Its all about creating a market niche and apple has done that with their macs. Its the same thing with the blackberry versus other smart phones. They aren't number one in the consumer end but on the business side you'll find almost no company that uses smart phones where their employees don't have a blackberry as they are the most secure smart phones and their business enterprise server adds to this security for businesses. The iPhone may be one of the more popular consumer phones but it is the least secure smart phone on the market.
Since we've had issues in the past with mac/apple threads getting out of hand this is the only warning anyone will be getting in here. Anything deemed inappropriate will see the user receiving an infraction