Why does imovie not have subtitle features like in clips?

Unfortunately, there is no way to change some of the more interesting animated titles and the theme titles.

See: https://help.apple.com/imovie/mac/10.1/?lang=en#/mov9c5e9641f

"Note: You can’t change the styles of some animated titles and theme titles."

Some of the predefined Title templates in iMovie allow a subtitle. Could you not use them? For example: