It’s ’cause gums are actually sensitive tissue and harder bristles makes them bleed unnecessarily. You can get just as good of a clean from using a brush with softer bristles.
I’ve tried the extra soft bristles and I find that they bend and look used too quickly. I prefer just the soft bristles.
That being said, everyone is different and to each their own.
If you like bristles as hard as porcupine quills, be my guest and enjoy your quills. I will stick with soft bristles. I love brushing my gums above the teeth line, so not even touching the teeth. It’s like a massage for my mouth. Try it! We use our mouths a lot. They deserve a massage or two.