I found an old thread where the admins were telling that the registry cleaner is an advanced tool that shouldn't be used without knowing what one is doing. It was proposed to move it under the "Tools" section, to not have it as a big, tempting button in the main view - I myself have also used it for ages because of that. One of the devs said that it's likely to happen in a future update. However, fast forward almost 5 years and it's still there.
From my UX Designer viewpoint the reasoning and logic behind this makes sense: if a feature is very visible and easy to use, people will think that they're "supposed to" or "can" use it , if they don't know any better. Especially with a cleaner software such as this, people will use it for potential benefits, but apparently on newer Windows it does the opposite in most cases.
