Because Minix is neither overly complex, nor obsolete. It nears the sweet spot between simplicity and functionality - I don't know how much of that I can attribute to its architecture. I don't think it has anything to do with academic credibility or holding grudges.
That's my opinion, of course, and perhaps my CS professors stuck to Tanenbaum religiously.
While Minix, like dozens of OSs, is far from having a measurable market share - especially compared to Linux/Windows/BSD/etc - is also far from being either unwanted or disused. It's actively developed and promoted. Why would you characterize it obsolete based on its difference from "mainstream" OSs? That doesn't make sense.
That's my opinion, of course, and perhaps my CS professors stuck to Tanenbaum religiously.