Because not everyone shares the same interests and they may not find their engaging connections here?
Not everyone in the world likes Facebook or Twitter or Tumblr or McDonalds. Not everyone has to like something for it to be successful.
Because the rewards for most users will be less than $5 per month, i.e. meaningless?
I'm not sure where you're from in the world, but $5 is a lot of money to a lot of people around the world. And besides, when a platform is consistently allows the publication of high quality content on the Internet, that's valuable regardless of if you're getting paid to read it.
Medium reports only 20,000 of its 25 million readers blog weekly. Blogging is not something everyone can do well. So what do the rest of the users do that is engaging?
I think you've answered your own question here. Why is Medium so popular, if only such a small percentage like to write on it?
Which would disincentivize cashing out at some level above zero, because of the cost of cashing out isn't 0.
Yes, agreed.
Have a good one