It's more expensive, it's closed off, there's fewer programs for it, it's not enterprise friendly, people aren't used to it, and it's more expensive.
Closed off as far as hardware choices yes, as far as software, no its more open that Windows is. Mac OS is built upon Darwin Unix. I also disagree with the "not as enterprise friendly". They work fine for us. Now if your place of work uses some custom in house apps then maybe its an issue but for almost everything the macs are actually better, much easier to set up on the network. We usually only ever have to deal with them when setting up new ones.
To the OP:
Price and myths are the two reasons its not as popular.
First, price:
People have this idea that "you're paying more for less" with a Mac, yet they fail to take into account the quality of hardware and OS you get. You simply can't find the same quality of machines in the Windows world. It would be like comparing a Lexus to a Neon. Sure both can get you to the same place but one is a much more pleasant experience.
Also people forget that Apple and Microsoft have different business models (Apple makes money from hardware, MS for software). This is where you get dumb people who talk about the "Apple Tax". One could easily say Microsoft has a "Microsoft Tax" since they charge so much more for their software than Apple does ($400 for the full version of MS Windows, $400 for the full version of Office, etc). The thing is NEITHER company has a so called tax, they have markup on different products and thats how the make their money. Apple is a hardware company, Microsoft is a software company. If you want high quality hardware in the Windows world you will pay almost the same as if you were buying a Mac.
Second, myths:
I can't tell you how many times I've heard people say things like "Well I heard Macs wont work with anything." Maybe back in the day that was true but not in todays world. I haven't came across a printer, hard drive, mouse, keyboard, scanner, speakers, cameras, camcorders, or other misc peripherals that I haven't been able to connect. Unfortunately for some reason people still believe that macs aren't as compatible. I find the reverse to be true actually.
I love being able to have multiple desktops, built in ISO support, the ability to create and resize partitions on the fly without a special program, etc. I can even run Windows on my Mac (which I do, I work with Windows for a living so it only makes sense for me to have it on my Mac).
Anyway those are the two reasons (there may be more of course) that people don't like Macs.
Also I could be wrong but I'm pretty sure Apple rakes in more profit than HP and Dell.