WrestleMania IV question

Why was Vince so intent on a tournament?

Been watching some of the syndicated 1988 shows, and it seems like a double main event of Hulk/Andre and Savage/DiBiase wouldve been a better draw.

And simple too. Jack Tunney can declare DiBiase as the recognized world champion because WWF rules didnt forbid selling the title (it wasnt a problem when he tried to buy it off Hogan), but he's changing that moving forward. Hogan could challenge Andre in lieu of a title match to settle that score, opening the door for Savage to get the shot. 

Let the rest of the tournament guys have single matches with some sort of build. Savage/DiBiase could tear the house down if they only had that one match. The show wouldve last less than 10 hours as a bonus. 

Is there something about needing a tournament that I'm missing?   

Nope.  The Observers were burying the idea at the time as well.  They had to do a Hogan vs Andre rematch of some kind either way because they were nuts not to.  This was probably the worst possible way to get there.