[EvilTerran]

Bluntly, because when we still had such things as sodomy laws and the "gay panic" defence[0], society wasn't ready to deal with that nuance in a productive way. You need to keep your message simple when you're trying to change deep-seated beliefs, lest people perceive a loophole in the minutae to excuse clinging to their prejudices.

Both the notions "anything but complete heterosexuality is unnatural" and "sexuality is a strict binary" were widely-held misconceptions in need of correcting, sure, but it was prudent to dismantle western sociey's attachment to the former before tackling the latter. If the idea of sexuality as a gradient had been popularized first, it would have been misused as a stick to beat down the campaign for acceptance of minority sexualities: "Why should we allow homosexual behaviour? As you've just been telling us, it's not a binary thing, so surely you're all a little bit straight? Sorry, you'll just have to put up with living according to only that part of your inner self. Stop complaining - according to you, we're all a little bit gay, and you don't see us grumbling about not getting to act on it."

[0] https://en.m.wikipedia.org/wiki/Gay_panic_defense