LDAP is typically a behind-the-firewall protocol. At that point, in the "old school" mindset, it's considered a trusted service. Having features to automatically pick up stuff across your own network of boxes might be considered useful by many an admin.

Also, my understanding is that Java deserialization (or deserialization in general) wasn't intended to explicitly allow actual code execution, just reconstitution of an object's state from storage on disk, the network, etc. Sometimes the state of certain types of object can be repurposed to result in arbitrary code execution, but AFAIK that wasn't an anticipated outcome or design goal back in the 90s.