1) Remembering 15-20 complex passwords is impossible. Not just "not easy", but impossible. You'd have to write them down.
2) You have no idea how many businesses have a black binder with a few pages of passwords in it. If the passwords are written down, they can be stolen, and they can't easily be changed without making consistent changes to your binder of passwords.
3) Shared passwords are not easily managed using a password manager "keychain service"
4) Some computers cannot have the password manager installed on it, ruling out its use (my biggest problem - I have a locked down computer that it is completely impossible to install 3rd party software on)
5) In the event that (4) applies, you could use the manager but then have a bunch of random passwords to remember, compelling you to do (2).
6) Sync between password managers. Not that it's impossible, but it has issues similar to any other networked application. Firewalling, filtering, proxies...
I'm not against the password manager as a concept, but we're a long way away from the common user being able to use it easily.