Because I mostly don't like web interfaces. It's not that Portainer is bad, it's not, it's really good actually, but I like software to show up as their own thing when I tab between windows.

I don't want yet another tab in my browser, I also don't use webmail, web based chat clients, IDEs, newsreaders and so on. I'm mostly okay with Electron apps, if their done right (VSCode and 1Password comes to mind a two really good examples).