Why Software Projects Fail ?!!

I don't know but every company suffers this problem why software projects fail , you know the famous triangle of cost-time-quality ; and cost includes both people and machines or some required software to assist or make delivery of the project faster , anyway I think that there is no utopian management even in MicroSoft itself with all the buzz of MSF , they postpone their projects for example Windows Longhorn , Whidbey , and all the open applications to be delivered , I attended a demo about Whidbey at MS Egypt and the presenter said that " we'll not deliver ObjectSpaces in this release !! " I don't know why even I told him that it's already done so why .. ! actually nobody knows what is going on behind the scenes so here I open this discussion to know why projects fail ?! you should say right based on your own experience and we'll disccuss ..Thanks!