Well, first off, Strongbox is Apple specific, and I don't use Apple devices. Second, it does not appear to be 100% open source, though it is based on some open source code from KeePass.

So the real question for me would be why not use KeePass, which is what I used before switching to Bitwarden. I switched because at the time, Bitwarden had better clients, better integration with Android and Firefox. That situation has changed some since, but I haven't had any good reason to switch back.