Verifiers SHOULD NOT require memorized secrets to be changed arbitrarily (e.g., periodically). However, verifiers SHALL force a change if there is evidence of compromise of the authenticator. (source)

I found an explanation on a different site:

It’s difficult enough to remember one good password a year. And since users often have numerous passwords to remember already, they often resort to changing their passwords in predictable patterns, such as adding a single character to the end of their last password or replacing a letter with a symbol that looks like it (such as $ instead of S).