People are lazy by default, they prefer simplicity and convenience. HTML compared to markdown has better results, but is less convenient and more complex. That's all what is there.
I used to write notes in HTML and for example creating list in HTML is annoyingly slow and tedious process, while creating list in markdown is just writing one symbol. Done.
Then you will get to create inline code or block of code, suddenly you need to write span (or div) pointing on a CSS class where you will specify formatting of that inline code. So you need to link CSS file or have CSS code in header of your HTML file. In markdown you will just use one symbol (or three for code block). Done.
I used to write notes in HTML and for example creating list in HTML is annoyingly slow and tedious process, while creating list in markdown is just writing one symbol. Done.
Then you will get to create inline code or block of code, suddenly you need to write span (or div) pointing on a CSS class where you will specify formatting of that inline code. So you need to link CSS file or have CSS code in header of your HTML file. In markdown you will just use one symbol (or three for code block). Done.