Why does a real IT landscape end up with a "colourful mix" of old and new IAM protocols?

Question

Accepted Answer

Because standards change faster than the existing IT: applications live for many years and keep speaking the protocol they were built with, while new applications arrive with newer standards.
A company does not replace all its applications every time a better standard appears. A payroll system bought fifteen years ago may still authenticate with Kerberos or LDAP, a SaaS tool bought last year expects OpenID Connect, a partner portal speaks SAML, and a cloud provider wants SCIM for provisioning. Replacing working applications just to modernise their login is expensive and risky, so the old stays while the new is added.
The result is a lasting coexistence (buntes Neben- und Miteinander) of old and new protocols and technologies. For IAM this means:

the IAM system has to support several generations of protocols at once, often through gateways or translation layers;
processes must feed the same identity into very different targets;
old protocols (with their weaker security) stay in use longer than anyone would like, which is a risk to be managed, not ignored.

Go deeper:

Legacy system (Wikipedia) — why organisations keep old systems running and what that costs.