As far as I know, I am neurotypical, so mind that when reading my comment.
Regarding the title of your question:
Because neurotypicals are the majority of people. As usual, majorities set norms and actively or passively decide upon common concepts like ethical or societal rules.
Regarding the issue you described:
Even though it might seem exhausting, I think it’s important to see people – regardless whether neurotypical or not – not as a homogeneous mass but as a highly variable mixture.
Sure, even then there are common rules like don’t punch people. But those are the broad and general ground rules.
Then, there are the nuances, which can be highly individual.
In your example, regarding the punctuation, use of grammar as well as verbosity of replies, I would react completely different than your friend. I wouldn’t mind any of those things, which seemed rude to her. Okay, maybe I would feel like I weren’t given the attention I hoped for if, after pouring my heart out, I just get a simple “K.” as response. That would make me sad, because I was hoping for compassion and a dialouge dealing with the issue.
But apart from such things, that would probably be totally fine for me.
One behaviour and two very different reactions due to two different people.
As such nuances are often individual, it’s probably best to explore what kind of behaviour the other person would feel comfortable with and with which behaviour they don’t. Repeat that process with everyone in your life, who you would also like to keep relations to.
That’s also related to a thing known as role-behaviour in psychology, which also applies to neurotypicals. When I talk to my superior at work, I behave differently than when I am at home with my wife. Then, I also behave differently with my friends. And among the friends, I learned enough about some, to know what and how I can say something to them and from which topics or phrasings it’s probably best to steer away.