It’s often due to the security department following outdated standards. Nowadays NIST recommends the following:

Verifiers SHOULD NOT impose other composition rules (e.g., requiring mixtures of different character types or prohibiting consecutively repeated characters) for memorized secrets. 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: https://pages.nist.gov/800-63-3/sp800-63b.html

That said, the company I work for violates all of the above rules …