Why are some historical figures known by their first names?

Some historical figures are colloquially known by their last names, e.g. Einstein, Shakespeare, Descartes.

But some are referred to by their first names, e.g. Galileo. Michelangelo, etc.

Is it just a matter of which name is the most unique and memorable?

Who decided that Galileo Galilei would be referred to in schools as Galileo and not Galilei?