Because the Mozilla brand is more trusted than a random website with ‘Pwned’ in its name. Also, it's being built into Firefox so having a website for it too seems like a good idea.
Well the have I been pwned website is also pretty trusted and is integrated into 1password. I don't know why Firefox wouldn't just integrate it into the browser like 1password did with their password manager. Would make more sense than just being a different front end to an existing site.
Pwned is not a standard english word. The vast majority of non-tech non-gamer non-under 40s are unfamiliar with this word but are familiar with firefox.
You think people of that age are familiar with what a "Fire fox" is? With all the people I've helped most don't even know a browser outside of the default on their system.
