Because it would go against the Chief's character; romantic feelings could have an impact on the team dynamic and unit cohesion, and he wouldn't allow something such as romantic feelings to cause unnecessary risks. Entering a relationship within the team would cause unnecessary risks and so would be off-limits in his eyes. The reclaimer saga is about humanising the Spartans, but the Chief is still a Spartan and he would know that a relationship could put lives at risk.
And you're going to back this up with anything substantial, or...?
Well, if they 'might', then it'd be a catastrophically written story. It won't be happening.
A possibility that won't come to pass.It might
Seriously. Why is two people who have cared about each other their entire lives, who trust, and respect each other in healthy ways possibly becoming a couple so scary? Especially if it's done the same way their relationship has been built up in things like the books up to this point? The execution of it is obviously important, but unless you can see into the future with 100% accuracy there is no declaring how good/bad said story would be written.
It would also cause a large part of the community to see Kelly as "just Chief's girlfriend", when we know she is far, far more than that.
Well we do know that the Chief doesn't just not feel things - so while it is logical to say that he and Kelly may not be in a formal relationship at this point in time, there is still no real way to say that there are no romantic feelings between them whatsoever. Those feelings becoming slightly more apparent in the future wouldn't be against either of their characters at all. And them being so close as is hasn't caused any unnecessary risks to the team thus far, so why would shifting their dynamic a little change that? It stands to reason it wouldn't. Also whose to say they don't enter into an official relationship once they're more or less in retirement (provided they survive to the end of their story arc)? There's certainly ways to do this that don't completely upset the status quo.
Again, if it's done in the same subtle manner as it always has been in the books, I don't see how anyone could just write Kelly off in such a way. Regardless though, why should the community's inability to process mature relationships prevent something that could be a healthy and positive development for both characters? That seems pretty counterproductive as far as the story is concerned.