I wager you already the answer. Without explicitly stating it, people will automatically assume everyone is heterosexual. More to the point, they will then tend to leap down the throats of anyone who attempts to speculate about the sexuality of a character by saying there is no evidence to support their potential homosexuality and that it exists only in the minds of those who wish for it to be there.
We see this quite regularly in WoW when people bring up Anduin and Wrathion for example.
So, it is the attitude of the people who don't wish to see it which makes it all the more imperative that character's sexuality is displayed...because those who don't wish to see it aren't in favor of 'neutrality'. As discussed, in a story where sexuality is left opaque, everyone assumes every character is default hetero. At that point, it becomes exclusionary.
As to why it is necessary to be done...well, sexuality is an intrinsic part of who we are. It is one of the defining aspects of our humanity. You cannot make a good character without determining their sexuality. That isn't even just a matter of who they are attracted to, but what quirks turn them on, what they are interested in...what they pursue. In the majority of cases, the answer will be the character is heterosexual. In a few though, the character will be LGBTQ.