The Internet is an insecure place. Many of the protocols used on the Internet do not provide any security. Thus, applications which send an unencrypted password over the network are extremely vulnerable.
Some sites attempt to use firewalls to solve their network security problems.
Most of the really damaging incidents of computer crime are carried out by insiders. In many places, these restrictions are simply unrealistic and unacceptable. Kerberos was created by MIT as a solution to these network security problems. This only ensures that your password is secure, not any transmitted documents or the text of your email. Like any other piece of software, Kerberos has its bugs.