So I guess your question comes back to "Why is the iPhone more popular than Macs?".
I think it has to do with expensive phones being more popular than expensive computers in general.
A smartphone's monthly bills are so expensive over time relatively to the device itself that people will not be cheap on the hardware side if they're going to spend thousands of dollars on monthly bills before the contract ends anyway. Also, carriers subsidize expensive phones so they are more accessible.
Think of it this way: If you had the choice between a 500$ MacBook Pro with a 2 year contract or a free netbook with a free 2 year contract, and monthly home internet bills were always around 200$ for everyone, which hardware would be more appealing to most?
Yup, people would buy more expensive computers in this kind of situation. Apple totally dominate the expensive computers market. The vast majority of PCs sold are under 1000$. It's common for someone to buy a 2000$ Mac. A PC? Not so much. The few that will do are mostly gamers that will build their own gaming desktop. And say, Alienware vs Mac laptops sales don't even compare.