There's a whole lot of space between hosted Wordpress and hacking on Jekyll. Not that hacking on Jekyll is hard, but there is something to be said for a more portable and easily installable/cross-platform solution. For example if my dad wanted to start blogging I wouldn't want to point him to Wordpress, but there's no way he'll dive into Jekyll.
Sure, but I don't think there are many people to whom "simple" means a command-line interface, or even a local generator whose files you upload somewhere. A hosted solution with a simple edit box is much more in line with most people's notions of simple. I think that's what Medium is going for.
