People use Medium because they want to be heard. Marketing is hard (although this author doesn't seem to have problems with that.)
Because when I switched to Medium my blog posts went from ~50 impressions to 1000+ impressions
Exposure.
Are you using a custom domain with Medium?
No I opted for no-domain because my articles can be sequestered to any publication where they will receive better exposure than if I had reserved it for my own.
Probably the usual - it was new cool and convenient.
I don't use medium, but also don't need much flexibility. The content is my focus and priority.