I think there are multiple reasons he picked Reigns. Reigns has a great look, perhaps the best look on the entire roster. He's family oriented, he wrestles a low risk WWE style, he seems willing to do whatever the WWE tells him without much, if any, resistance, and he taps into the women and children demographic. Vince essentially grew this guy in his basement from the ground up. The only experience Reigns has as a wrestler is within the WWE system, meaning he functions the way the company wants him to function as a performer. He was part of a hot stable that got over with the fans, and of the three men in that stable he was the most reliable and bankable looking of the three. He has heritage within the business, which means as much as WWE hates looking back at history because it makes them feel old, he has a legitimate pedigree they can fall back on in the form of the Anoa'i/Miavia family union.
Let's be a little fair to Roman here. There have been far worse legacies in wrestling that have been pushed with way less to warrant their ascension. He's not David Sammartino, he's not Erik Watts, he's not Jeff Jarrett, and he's not David Flair. Roman can wrestle. Roman has a great look. He's not simply there because of family, although it does sweeten the pot. He got over on the merits of the Shield, and once the faction split, the mystique he built up as the brooding quiet member of the faction was dissolved the smart fans realized he wasn't someone they wanted to get behind as hard as they had Seth or Dean. Casual fans still eat Roman merch up though, and that's where the real money is. It could also be raised that Roman's way too pliable as a talent, and to an extent that's been just as much a detriment to his career as it has been beneficial.
Let's also be honest with the fact that Roman really hasn't been a main event player for almost a year now, and in some ways that may actually be hurting WWE's ratings. Whether it be WWE becoming spooked by Roman's health scare last year, or them simply wanting to try and restart his build with his triumphant return from a battle with leukemia, the gas pedal's been let off of on the Roman push. They seem to want to get back to that initial intrigue by putting him with an equally stoic Undertaker in a tenuous tandem with a horrible name that will probably sell t-shirts.