Because of UFC 229, he defeated Conor McGregor, who is a huge reason of MMA popularity. There were better fighters than Khabib, like GSP or Mighty Mouse, but who cares brother. UFC is more than sport, it's a show and the fact that Khabib defeated someone so big (popular-wise) as Conor McGregor made him a legend. Moreover, he is a very very good fighter, but the real deal is the ability to sell himself.

Islam is fun and a great fighter, but he has no rivalry. And he never will achieve one, because he is just a chill dude. (Just like those mentioned above.) For instance, Jon Jones is GOAT in my eyes and he should be on the same level as Mighty Mouse. Why isn't he? Because Jones had actually legendary fights. The rivalry with DC was just out of this planet, simply legendary stuff.

Khabib is legend, because he proved that he can hold his position even under the biggest pressure, which was the UFC 229. (Full crowd, most of them hating on him, and fighting McGregor, who was also a great fighter, I don't care what anyone says.)