Because it’s closed source, there’s a higher likelihood that there is an undiscovered vulnerability in 1Password. Even though it is audited, a vulnerability could be introduced after the most recent audit and you would never know.

For something as mission-critical as a password manager, going with an open source solution gives just that much more confidence that your data is safe. To me it’s simply not worth the risk to blindly trust a company with my login data, when I could trust a company that displays their entire solution in the open.