#4 Shouldn't - It's too early
The most obvious reason why Bray Wyatt shouldn't challenge for the Universal Championship at Hell in a Cell is that it's too early for The Fiend to get a shot at WWE's biggest prize.
Every Superstar needs to earn a Universal Championship shot and frankly speaking, The Fiend hasn't earned that opportunity yet. He has only faced Finn Balor up till now and WWE should consider having many more matches before giving Wyatt such a big match.
Giving him a Universal Championship shot so early would be a huge gamble. Vince is known for making some irrational decisions, this decision of giving The Fiend a Universal Championship match could either turn out to be a genius move or a rash one.
#3 Should - Hell in a Cell is the perfect breeding ground for a character like The Fiend
Whenever someone has mentioned WrestleMania in front of you, The Undertaker would have definitely popped up in your heads. The same could be done with Hell in a Cell and The Fiend.
The two gloves you would have seen Wyatt wear has something written on them; it's hurt and heal. Now, Hell in a Cell is known for the hurt rather than the heal. So, everything sits well with the character of The Fiend for him to be made a dominant force at Hell in the Cell.
Inside a steel cage, The Fiend can be displayed as an unstoppable force. He could possibly go on an unbeaten run at the pay-per-view, just like The Undertaker did at WrestleMania. This could be another reason why Vince is tempted to give The Fiend a huge match.