#4. He feels that Strowman doesn't "need" to win a championship
There are only a handful of superstars who are larger-than-life and have characters that never really require any titles. Going back in time, some characters for example, include Roddy Piper, Jake "The Snake" Roberts and even The Undertaker, though the latter found a lot of championship success.
Bray Wyatt felt that way to some extent as well and Vince McMahon likely feels that Braun Strowman belongs in that category. Being a massive superstar physically, he may feel that Strowman is an attraction in itself and doesn't necessarily need to hold the World Championship or Universal Championship to feel special.
Unfortunately, what he doesn't realize is that by missing out on multiple opportunities to crown Strowman, he has severly affected his momentum. Strowman needs a big title win at some point in the near future, otherwise he'll continue to suffer.