> Then why did HTML became so popular if win32 or MFC were so great?

One of the factors is that web dev pushes for a complete separation of concerns, and thus allows frontend developers to specialize in front end development. Therefore it becomes far easier to hire someone to do frontend work with a webdev background than a win32/MFC background.

Number of applicants is also a big factor. There is far more demand for webdev than pure GUI programming. You can only hire people who show up, and if no one shows up then you need to scramble.

Frontend development is also by far the most expensive part of a project. In projects which use low-level native frameworks you are forced to hire a team for each target platform. Adopting technologies that implement GUIs with webpages running in a WebView allow projects to halve the cost. This is also why technologies like React Native shine.

Also, apps like Visual Studio Code prove that webview-based apps can be both nice to look at and be performant.

It's not capabilities. It's mainly the economics.

In the win32/MFC days, there was no "front-end developer". There was only HTML and content creators writing it.

Then there came small web applications, and still no "front-end developers", since functionality could only work on the server.

It's only when AJAX was introduced in the mid 2000's that you could start to talk about "front-end developers".

By that time, win32 and MFC was old. We had Java, C# with .net framework, etc.