Microsoft has negligible presence in the high-margin mobile and tablet space. By contrast, Apple makes hundreds of dollars from each phone/tablet sale. Phones are repurchased every 2 years, and Apple originally got $18 per user per month for each iPhone subscriber (http://news.cnet.com/8301-13579_3-9803657-37.html). Microsoft makes ~$50 from one-time PC purchases, which are replaced every 4-5 years.