Let's examine the question here: Why not? Totally ignoring the EU, which nobody pays attention to until they really need to win an argument, what the Hell would it hurt? It would change exactly nothing about what makes him cool or why people like him. It would make him no less quotable or iconic or interesting as a character. "The Force will be with you, always," would not take on weird connotations unless you're the kind of person who fixates on sexuality you consider deviant.
I chose Star Wars and Obi-Wan for preamble to this and most subsequent example-heavy arguments for several reasons, the most important of which being that I've actually seen the question posed. On the subject of gay people in Star Wars, someone asked on a forum why Obi-Wan couldn't just be retroactively outed like Dumbledore. And yeah, while I'll agree with other people who know far, far more about Harry Potter than I do and say that outing him at a press conference after the series was over wasn't the most exciting or world-shaking way to do it, the basic question of "Well, why the Hell not?" remains. For another thing, unless you're counting the EU, romance and sexuality really isn't addressed in Star Wars. Yeah, I know there's Leia and Han, and Padme and Angrykin, but beyond that it's not really touched upon outside the EU where everyone who matters needs to have a girlfriend.
It was pointed out to me by a perceptive friend that there's definitely a prevalent stigma against the notion that iconic or otherwise cool character could be anything but heteronormative, and I'm inclined to agree. I know I've nattered on for a few paragraphs about the fact that sexuality doesn't actually affect how cool a character is, so maybe you're thrown by my getting all irritated at every worthwhile character in mainstream media being straight. Well, this goes somewhere beyond a character being cool or memorable.
I don't know who reads this. I don't know if anyone does. I don't know if you're gay, straight, bi, trans, whatever. Being abnormal, especially as a kid, fucking. Sucks. Not regular abnormal, like... un-normal. Deeply and fundamentally different in a way that you don't see in your peers, in your family, or even on TV. Surely some of you remember how badly you wanted to know people like yourself as a kid; I don't doubt many of you feel that even now. We are social beings, after all, and we form our opinions of ourselves based on the way we perceive our world. Growing up in the eighties and nineties, when English-language media decided that every children's show needed a rainbow of ethnic cast members, plus a wheelchair-bound kid and a tomboy, still managed to be isolating and lame. The tomboys, by and large, still wore pink shirts under their overalls and inevitably developed crushes on nice, normal boys. Boys with gender-inappropriate interests were usually one-off characters, which made me even more bitter and confused. Why didn't the wheelchair kid ever just get up and get over being crippled? Was I eventually going to get over feeling like this? Am I just not worthy of attention, like that random femmy boy we saw for one episode and promptly forgot about? WHAT?!
Any covert attempts on my part (futile as they were, as I grew up in the deep south in the middle of nowhere) to find queer characters ended in enormous disappointment. I wouldn't go so far to say that every queer character I managed to find was miserable, but they were definitely happiness-challenged. they were sick, or they were riddled with angst, or they were persecuted, or any combination of those factors. Did I mention that every single piece of media I found was ABOUT how sick/persecuted/miserable they were? Unless they were evil? Because that was the case! I didn't want queer characters who were villains or miserable douchebags defined by their sexuality, I wanted queer characters who were like the other characters, only abnormal like me. Even when I got recommended the Herald Mage trilogy by an older friend in high school, I wasn't satisfied because Vanyel was so fucking whiny about his sexuality despite pretty much everyone else in the book's world NOT CARING.
It's fucking mindbending to look for someone to relate to in terms of sexuality and still look up to them. I eventually just started looking for subtext, and that was pretty damn helpful.
I don't think every character should be gay, or bi, or anything. That would be an unrealistic expectation, and frankly pretty weird. But it's just as weird when you realize that in many works every single character is at least implicitly heterosexual. I don't get why people choke back hurl when they're subjected to gay kissing onscreen while queer people have been thoroughly desensitized to straight kissing through years of exposure. Why the Hell shouldn't everyone else have to sit through romances they're not personally invested in? Maybe they'll wind up like me and care less about the sexes of the people involved and more about how poorly written or compelling it is.
Yeah, that's a weak ending but what the Hell do you want on a blog made for rambling until I've vented all my vitriol? Time to replace it with beer.