I've thought about this constantly. Sure, he's lyrical, but so are a lot of underground dudes. His beat selection is hit or miss compared to guys who are well known for selecting hot beats, so it's not that. Maybe it's his ability to connect to the listener with relatable material. He's not claiming to be a gangster or a drug dealer, etc.
I think above all else, it's his versatility. I thought that line on Rap God "throw on Lose Yourself and make em lose it," was a sharp line. If you look at it the way I do (which may be a reach), but he can get all serious and motivational (Lose Yourself) and then get silly (Just Lose It) and his credibility is in tact.
When he's angry, we believe him. We take him seriously. When he makes videos dressed in drag, we laugh.
Other emcees have boxed themselves in, in my opinion. They get too caught up in their image and it limits their creativity. They decide, I'm gonna be a hard rapper, or a club rapper, or whatever it is they choose to be, and that's it. Anytime they break away from that, it comes off as try hard, or weird. They're stuck being one note artists and eventually their material gets stale.
I think Eminem's versatility is the reason he's outlasted many other emcees. The reason he's been this popular for this long.
What do you think sets him apart from his peers?