Why did the staff choose to make new citizen ID card items instead of simply reinstating the old ones? I was under the impression that the old cards also changed your homeworld permanently. Were they permanently retired and their effects deleted from the code, necessitating a new item altogether? Or did staff just forget about the old cards?

The older planet visas were intended for different functionality originally - not for changing planet residence. They are actually fairly early in item history - likely v1 or v2. They may or may not present an opportunity in the future to become useful again so we made new items instead.