The answer above compares Magnum T.A. to two mid-card WWE comedy performers?!? There is no way anybody who really saw Magnum T.A. wrestle would make that comparison.
The first answer comments that Ric Flair could make any face a star. That's pretty much true. Flair was such a consummate heel that any face working with him became a crowd favorite. But to say Flair is solely responsible for making Magnum T.A. a star is selling Magnum short (not that the first answer did).
I started watching Magnum in the very early 80's, before he became a huge star. He was one of those few who "had it all". Excellent wrestling skills, could brawl very well, very good-looking (but not a "pretty boy"), excellent size and physical shape, loads of charisma, good mic skills (though not at the same level with the great promo-ers like Flair, Dusty Rhodes, Tully Blanchard, Jimmy Valiant, Terry Funk, Jim Cornette, and Gary Hart), and maybe most important of all, he was believable in everything he did; he never came across as an "act".
There is a huge difference between Hogan and Magnum. Hogan was a gimmick, a character, a "sports entertainer", a TV and multimedia star. Magnum was a professional wrestler. Hogan was created, pushed, and marketed to be a larger-than-life TV superhero, a role model for children, the ultimate "good guy". Magnum was a professional wrestler who wrestled for a living. He was a good guy who played by the rules (as much as possible), personable and friendly, and most fans liked him. He wasn't a gimmick or a character or a superhero. He was a hero to the working class, much the same way Bruno Sammartino was.
No doubt had Vince McMahon been able to sign Magnum T.A. he would have made, or rather, maintained Magnum at his level of stardom, and possibly exceeded it. But Vince would have hung a WWE gimmick on him and had somebody write his promos. He wouldn't have been "Magnum T.A." anymore. Because Magnum looked and talked "southern" Vince likely would have given him a cowboy gimmick. Magnum would have made it work but it WOULD change what he was.
But assuming Magnum didn't have the car wreck that ended his career and he did beat Flair for the NWA World Title as planned and assumed the top spot in the NWA from Flair, did he have the talent, charisma, and personality to take HOGAN'S spot at the top of the wrestling business?
No. Oh, he was a very likable guy and could wrestle rings around Hogan, but the nature of the business would have kept Hogan on top. The NWA was "southern rassling" that appealed to, and was watched by, "rassling" fans. The WWE was "sports entertainment" for the mainstream. At the time, only Flair (in the NWA) had managed to edge into the mainstream, whereas quite a few of the WWF's performers were becoming (or already were) household names in the mainstream. Magnum T.A., as good as he was, was not Ric Flair, the bad boy everybody loved to hate. And he certainly wasn't the larger-than-life superhero Hogan was. Magnum would have been an extremely popular NWA World Champ, at the level of the rest of the NWA World Champs in fame and recognition (except for Flair).
Basically it came down to what the (global) mainstream audience wanted. And they wanted the larger-than-life superhero. Flair had Hogan beat in every department except for charisma (if Hogan is 100 on the charisma scale, Flair is 99). The global audience still preferred Hogan. It would have been the same with Magnum as the "face" of the NWA. To make a modern comparison (for popularity and level of stardom), Hogan is Cena, Magnum would have been Orton. Magnum was an excellent wrestler and brawler, charismatic and very likable, and he was good on the mic. But he wasn't a larger-than-life superhero.
Nothing was bigger in the wrestling business in the 80's than Hulk Hogan and Hulkamania. Nothing could have been. Flair gave it a good run for it's money but he WAS a distant second place. Magnum would have been, too.