1. Microkernels failed
For the reasons that Linus Torvalds stated, that on paper it looks theoretically attractive but in implementation on modern systems - which are very complex systems - the complexity becomes exponentially unmanageable. Case study is GNU Hurd, a fully microkernel system which failed to even achieve basic functions. Mac OS X is similar to Hurd in structure and it's the least stable and most constrained OS out there.
2. CPU architecture
This has become diversified for various use cases. One CPU architecture did not dominate because embedded, mobile, desktop, server and so forth use cases are different and required different approaches. Tannenbaum failed to see this diversification.
3. GNU vs World?
GNU did not dominate, but Linux did on server, embedded and mobile. Apple tablets and phones run iOS which is just plain old Unix. Accurate statistics are difficult to acquire for Linux deployments on desktop because there is no real core mechanism - sale of units - that can surely give an accurate value. Most Linux deployments on the desktop are sometimes recorded as Windows deployments because users buy a Windows system and then write over it with Linux. However, if you segment OS's then Linux has around 5-6% on the desktop according to http://www.w3schools.com/browsers/browsers_os.asp and this is equal to the number of Windows Vista users worldwide which is highly significant.
Based on my own estimations from various sources it seems that Linux on the desktop could actually be equal to the number of users on Windows XP - roughly 25% - if you count non-Western nations like China and India where Linux is more popular than in the USA or EU but who might not be counted in Western statistics because they only count traffic to English speaking websites aimed at Westerners.
In India most college students use Ubuntu or Fedora because this is the default OS of the Indian education systems and at the famous IIT's. Most Indian government offices use Linux also. In China Red Flag Linux is the offical OS of the Chinese government and school systems - Academies of Arts and Sciences - and is the recommended OS in China by the state run media as an effort to stop young impoverished Chinese from using pirated copies of Windows. If you counted the useage of Linux in India and China it would shock most Western tech experts and radically change the perceptions of the true dominance of Linux desktop in non-Western developing nations where it is dominant.
