Wix is targeted directly at small businesses who create their own web presence by using a template and filling in some details (text, images, etc).
Webflow is a much more professional tool targeted at web designers, freelancers, and agencies who create custom responsive websites, usually from scratch (or from a professional template as a starting point). That is, people who create websites for others. To be successful with Webflow, our users need to think somewhat like a developer (e.g. understand the box model, understand how CSS works in principle, etc), so the learning curve is a bit steeper.
For example, everything you see on https://webflow.com/ is built visually in Webflow and deployed to staging and production by non-engineers. There's a full database there with over 20 tables representing different content types, with a complex schema (e.g. blog posts, case studies, testimonials, job posts, templates, tags, etc), all the pages are custom responsive layouts - and all this was created without writing code.
None of this would be possible with Wix, Squarespace, Weebly, etc - since those are limited to their available templates. Most of the sites created in Webflow (see https://webflow.com/discover/popular for some examples) can only be recreated in Webflow or by writing code by hand, no other visual "WYSIWYG" builder comes even close.
You can think of Webflow as a much more evolved Dreamweaver, or a purely visual next-generation WordPress - minus all the code :)