Because they work at a standard level of abstraction (the file system) and can be replaced if necessary. I can replace Dropbox with Box or Syncthing or OwnCloud

Because they work at a standard level of abstraction (the file system) and can be replaced if necessary. I can replace Dropbox with Box or Syncthing or OwnCloud or rsync or sftp coupled with crontab or an FSEventsD monitor or whatever I want, at any time I want. 1Password's proprietary sync isn't open source and isn't open standards. It is designed to lock me into reliance on a company that has inarguably demonstrated that it hates its users.