Why is the font element not supported in HTML5?

HTML tags describe their contents. To mark up your HTML you should use CSS. In such way you can create different skins for the same information. Since the font tag serves purely visual purpose, it has no place in HTML; it does not describe the contents.

With describing contents I mean: "This is paragraph of text / this is a time / this is a link to another page / this is the title of this article / this is a list of related terms / this is a table containing test results" etc.

Read up on semantic HTML, it will hopefully make things more clear for you.

In short:

HTML is a markup language. Use it to mark up the different sections of your content using the most semantically accurate element.

CSS is used for styling purposes, such as changing colors, sizes, fonts, etc.

The <font> element was used for styling, not marking up a section of your content. That's why it's deprecated, along with other out-of-place elements such as <center>.