1Password used to ship native (aka "Mac-assed") apps. They (relatively, in the software's history) recently switched to Electron instead of continuing native app development.
So again, how does an Electron bug become 1Password's fault?
It's cross-platform and integrates with browsers so it makes sense they would want to use a cross-platform JavaScript solution as much as possible. Not just to make their developers more efficient, but to reduce the surface area for bugs and vulnerabilities.
Personally, I use it as much for other secrets as for browser passwords. Social security numbers, software licenses (not so much anymore), password reset questions, passwords I can't paste (for work), etc.
I don't use a plugin. Never tried it, simply never mattered enough (and I generally store frequently-used browser passwords in the browser's keychain as well).
It's cross-platform and integrates with browsers so it makes sense they would want to use a cross-platform JavaScript solution as much as possible. Not just to make their developers more efficient, but to reduce the surface area for bugs and vulnerabilities.