I think Apple is destined to an eternity of low market share. I don't see any way of digging out of it. But I also don't see that as being automatically meaning doom for Apple. Sure, I'd love Apple to start chipping away at market share, just like all of us, but what is just as important is profitability, which they have delivered in spades recently (last few years).
Yeah, 2% market share is frightfully low, but nonetheless it still represents millions of dedicated users, most of which will give up their Macs when they are pried from their cold, dead fingers. Apple is very strong in niche areas, which helps a lot.
But to answer the question, why is Apple's market share so low, we'd first have to go back twenty years when Apple (actually it was mostly Jean-Louis Gasse) had a choice to make..market share or profit margin. They chose profit margin and their shareholders were quite happy. So nearsighted.
So when customers are faced with good but expensive, or good enough and cheap, they generally choose good enough. The classic VHS versus Betamax is always the great example that comes up, and it's a good one. VHS was good enough, and more importantly, it was good enough in quantity.
If there's anything I've learned from this mess it's that the number one goal in technology development should be to get your product to the masses. I fear Apple is on the verge of making the same old mistake with the iPod. Yeah, they're selling great now, but Steve himself has said it's all about profit margin on those iPods. That's great for shareholders and quickfix money boost, but what about five years from now when a ten dollar Dell iPod that's good enough exists? And it's not iTunes compatible. I would think it would be Apple's best interest to get as many iPods on the planet, whatever it takes. But now I'm getting off track.
The other reason Apple's market share is low and has no hope of changing is in general business arenas like banks, hospitals, offices w/ large numbers of employees, etc. It's just too big an investment to switch over an entire infrastructure. Or too big a cultural change if nothing else. And as much as I freaking hate M$ and Windows, you can actually get work done on the things if you have to. There would have to be a compelling reason to switch and an office of, heh, Office users has no partiicular need to switch. It's not like they're editing video, managing music/photos, or looking for the subtleties of a refined OS all day.
And then there's the gamers. I think it's no exaggeration to say that if Apple could drop ten million dollars on R&D for a KILLER game (Mac only of course), you'd see a few percentage points switch. That gamer crowd is like the NRA man, there's just no compromise in those people. They'll drop a thousand bucks to get ten more FPS that they can't even notice. They're insane, god luv em.