Stuart Schechter makes some good points on the history of bad password policies:

Morris and Thompson’s work brought much-needed data to highlight a problem that lots of people suspected was bad, but that had not been studied scientifically. Their work was a big step forward, if not for two mistakes that would impede future progress in improving passwords for decades.

First, was Morris and Thompson’s confidence that their solution, a password policy, would fix the underlying problem of weak passwords. They incorrectly assumed that if they prevented the specific categories of weakness that they had noted, that the result would be something strong. After implementing a requirement that password have multiple characters sets or more total characters, they wrote:

These improvements make it exceedingly difficult to find any individual password. The user is warned of the risks and if he cooperates, he is very safe indeed.

As should be obvious now, a user who chooses “p@ssword” to comply with policies such as those proposed by Morris and Thompson is not very safe indeed. Morris and Thompson assumed their intervention would be effective without testing its efficacy, considering its unintended consequences, or even defining a metric of success to test against. Not only did their hunch turn out to be wrong, but their second mistake prevented anyone from proving them wrong.

That second mistake was convincing sysadmins to hash passwords, so there was no way to evaluate how secure anyone’s password actually was. And it wasn’t until hackers started stealing and publishing large troves of actual passwords that we got the data: people are terrible at generating secure passwords, even with rules.