"Apple's break with IBM stemmed from Jobs' wish that IBM make a larger variety of the PowerPC processors used in Macintosh systems. IBM reportedly balked because of concerns over the profitability of a low-volume business. By wrestling away Apple's business from IBM, Intel tightens its stranglehold on the PC processor business."
Read the article
here.
Other articles mention that reasons include Intel's support for a hardware-level copy protection technology (Trusted Computing Platform Alliance) and Apple's hollywood appeal.
http://news.com.com/Apple+to+ditch+I...3-5731398.html
http://www.wired.com/news/mac/0,2125...w=wn_tophead_1