So, I think there may be a few things at play here.
First, the competition is not running MacOS X.
Second, and it's unfortunate, but due to the world economy, people with less money than others are not spending it. People with more disposable income I think tend to gravitate to higher quality products, therefore buying Macs.
Third, regardless of income, economy, and other factors, I think a large number of people are done with substandard products. The loss leader cheaply constructed junk just to save a buck isn't selling like it used to. People don't want to waste time and money (even if they have it) replacing items every other year, and so folks are starting to say no to that practice.
Fourth, the typical PC user (and big box store salespeople) are all about specifications. GHz, MB, TB, "blu-ray", HDMI, whatever... I'm guessing that most of them don't understand the ultra thin form factor.
The Zenbook doesn't look like it's cheaply made, but I haven't seen one in person. Of course it's also running Windows, so there's one strike against it. But, they need to be patient; things don't usually just 'take off' over night. Even Apple sold very few of the original MacBook Air laptops. Asus and the others are going to have to resist the temptation to 'pump it 'n dump it' if they want to be successful in the ultraportable market.
Most "switchers" I'm acquainted with are just plain tired of their "PC" experience. Some of them are leveraging their iDevice to revisit the Mac platform. Some, like the waitress at my local favorite restaurant, saved up tips for almost a year to buy a MacBook Pro, and gushed at me the next time I was in the restaurant how much she loved it. Interestingly, even though she hadn't budgeted for it, she also purchased both the AppleCare and the One-to-One training as well. They set it up with her at the store after the purchase, so it was fully operational before she left. Her first session in One-to-One that same week they transferred her data and started her out on the adjustments from PC to Mac. She is one happy customer! I asked why she would pass up cheaper computers for the Apple, and she said that she wanted something that had better value, and that she anticipated owning the MBP for quite a few years. And that all started because her boyfriend bought her an iPod Touch I guess. But she loved the personal attnetion they gave her during setup and training.
So people are willing to spend hard-earned dollars for something they see as having significant value for the price, even people with little money - like this waitress - want something of better value, and will wait until they can afford it.