Why use this when Webmin has done the job for decades?
"X exists so why would anyone ever build Y". Why not? Competition is good, think about it for a minute and I'm sure you'll figure out how.
People are allowed to create their own X even if thousands of options already exist.
Second: webmin also has a patchy security history. I don't know enough about cockpit to say it's any better but it would certainly be enough of an issue to review all of the options.