UFC is a real sport based on Mixed Martial Arts, which combines striking with grappling and submissions.
A lot of WWE fans stopped watching the WWE because of the scripted storylines that make the sport seem fake.
A lot of people prefer watching actual fights over scripted ones.
A lot of people prefer watching fights that have blood in them. It is a part of why they watch the UFC and Boxing matches.
Another reason as to why people prefer MMA is because it is bringing the old luster of boxing back.
The WWE has suffered greatly from the PG-13 era that was aimed to attract kids. The UFC witnessed a great rise in views during that time.
A few of the big names of the wrestling world moved to the UFC along with other MMA organizations like Bellator; big names like Brock Lesnar and CM Punk.
A lot of people stopped watching the WWE and moved to the UFC because of female objectification where females had to have specific looks and shapes to be part of the WWE.
Ronda Rousey turned heads wherever she went and her rise in the UFC was incredible as well as her fall in recent matches. That was another reason to believe in the legitimacy of the female divisions in the UFC.
Trash-talking has been a huge part of MMA ever since its beginning, which has led a lot of fans to forsake the scripted trash-talk in favor of trash-talkers like Chael Sonnen and Conor McGregor.
Conor McGregor has been a huge star in the UFC and has brought hundreds of thousands, if not millions of fans, to Mixed Martial Arts.
Old wrestlers took the spots of new upcoming names that haven't had the time to develop into the superstars they could be. That is one of the biggest reasons why fans turned from the WWE to UFC where fighters are often young and fresh.