a) Battery life. For the most part, shoehorning Linux onto a PC is going to negatively affect battery life.
b) Despite being a bit of a "hacker", I have a day job and don't want to spend too much time tinkering with my OS. OS X is more polished than Linux and is less likely to go wrong.
I'm guessing that while most HN'ers aren't in exactly the same situation as me, they have more important things to do (i.e. working on their ideas) rather than wrangle with Linux.
I'm sure people will immediately chime in and say that Thinkpads work well with Ubuntu or that System76 does the job, and I'm sure it does, but the experience of running OS X on Apple hardware is simply unmatched.
>the experience of running OS X on Apple hardware is simply unmatched.
Eh. Seems a lot of people on HN believe this, but it's not always the case. Just a quick visit to discussion.apple.com will show all sorts of issues people have with the OS including bugs like http://www.cnet.com/news/some-lion-users-plagued-by-black-sc... which is a bug in the OS.
Edit: just some things I noticed in the short time I used OS X: The gmail integration is buggy. I didn't get messages on time, and I have to either quit the app or take Mail offline and then online again before new mail will start streaming in. Sometime I couldn't get it to quit.
The 'quicklook' thing seemed really slow, which seemed to be a problem a lot of users had.
Multi-monitor support isn't that great. I had multiple windows on multiple desktops for some reason and it just felt weird.
I had issues with audio stopping and not working again until I did a reboot.
In some apps scrolling was really rough.
Apple seems to think I should unitask on a single monitor. That's not how I work, so it's not for me.
People are saying they have to spend a lot of time configuring Linux but in my experience that's not the case (obviously once it's installed in the case of Arch, which for me is a worthwhile investment).
I do admire the polish of OS X's desktop though.
Now for hackers, you already get a long way with the built-in BSD commands, but one word : http://brew.sh
Once you've done that, you could install Linux or Windows or FreeBSD on it, but if you don't have an overwhelming requirement, you'll keep it on Mac OS.
And if you're like most hackers, you have one work laptop and you carry it around everywhere, and it's your primary machine.
Personally, I don't carry a laptop back and forth: I have a Linux desktop at the office, and a Linux desktop at home, and a Mac laptop for travel and moving around the house.