I don't think the issue is that "you need to speak your mind more often" applies to more situations than Radical Honesty does. Radical Honesty is a system that you can live 16 hours per day. It might even debatable whether you can do it for the full 24 hours.

The issue is rather that it takes more effort to understand Radical Honesty. "you need to speak your mind more often" is the kind of advice that a 5-year old can understand and is valued by the ELI5 crowd. And that sentence is quite ironic given what Radical Honesty is about (eg. you are not 5-year old and are therefore much harder to teach).