Tennis is a very rigid sport with a set of rules that makes it rather hard to be yourself on the court if you don't fit into the standard that the sport preaches. Kyrgios never fit and he always kept doing his thing despite tons of criticism from the outside world. That allowed him to remain a strong personality in a sport that hasn't got many of them.
According to Tursunov it's the key to his popularity:
"Players are afraid to show their personality, to say their minds, because nowadays anything you say can be twisted. You say that, 'I played great today,' someone's going to say that, 'Oh, he sounds cocky, he's putting his opponent down.' So players in general are just afraid to be themselves."
"That's why I think Nick Kyrgios sells out every freaking stadium that he plays in because he hasn't been neutered enough. He's been fined enough, but he doesn't mind it. He's a very strong personality of you ask me."