It's perfectly malleable - I can make every page do exactly what I want it to. It's lean and fast - every page even with all the interactive plots, games and quizzes, and syntax highlighting, is still under 64K. And it doesn't accumulate technical debt. There are no dependencies, even within the project. An occasional bug from the past wouldn't bite me in the future.
At the beginning, I thought that typing the HTML by hand would be too tedious. But it's not. It is a bit boring, sure, but it's only like 5% of all the work so usually I don't really want to automate it.
And when I do, I just write scripts that handle HTML just like any other text. Works wonders for me.