The simple answer is because of the new FSE (Full Site Editing) capability that has been included in this update. This new mechanism to configure the editor enables a finer-grained control and introduces the theme.json file which consolidates the various block editor APIs into a single point in the root of the theme directory. The documentation states that there is backward compatibility with "add_theme_support" which has been used by many themes up to this point, but my findings from updating 50+ WordPress sites today are that this is not always the case, and 'editor-font-sizes' seems to be replaced with the default settings