When people ask this question, all it really shows is that you are either completely out of touch with reality, or you are so far down the partisan rabbit hole that literally anyone criticizing the guy you voted for, must somehow be irrelevant. 

So, lets break it down. Eminem is one of the most successful music artists of our time.  Especially coming up in the Rap/Hip Hop industry.  So, he has A LOT of fans.  Millions of fans that have been with him for decades.  So when he not only criticized Trump but rapped about it, it made headlines.

For an alternate point of reference, Ted Nugent OFTEN criticized Obama and created entire songs about how much he hated him.  And when he did those things, THOSE made the news also!  

So, while you might not personally be a fan of Eminem, or care what he has to say, millions of people do.  WHich is many more people than care about what you have to say about anything.  So, thats why its news.