The U.S. personal-computer market declined for the first time in a decade last year, hurt by sluggish consumer spending, supply shortages, and the popularity of smartphones and tablets.
If you have a small profit margin, you're expecting growth but even then you're having to expend a lot to get that growth and you're extremely open to risk growth slows or declines. You have no financial buffer and it's a tougher hill to climb back up.
If you have a good profit margin, and growth slows in a quarter as happened with Apple and iPhones at the end of Q3 2011, you can pick right back up.
In response, PC makers are following Apple's lead as they did with tablets (mimicking the iPad) and are focusing on Ultrabooks (mimicking the MacBook Air) to drive growth moving forward. Apple placed huge bets on the future of technology and won. PC makers on the other hand are betting that the demand and markets that Apple created will translate to sales for them. There's many factors which place into it such as 'Do people want to run Windows?' which make it easier said than done.