I'm a scientist. Sometimes researchers send me precious, rare, one of a kind molecules and I get to work on them.
They don't get them back.
If I obtained a sample of moon rock from NASA for chemical analysis, once I was done, they would never get remaining sample back, unless specifically requested by formal letter or direct contact with the person who sent them, or unless it was specifically required that I return remaining material after experimentation. The sample would be kept and stored and I tend to track the paperwork of irreplacable materials.
The sad part about this situation is that once the records and paperwork and history (think, chain of evidence) on these rocks are lost, their scientific value is markedly less.
NASA is just an organization of people. Many of them scientists. All of them are friends and collaborators with other scientists. And the material just sorta flows around from person to person, then 40 years later they're all dead or retired and no one remembers anything.
Not surprised. Its not like there was a Rock Czar overseeing with religious fervor who got which samples of what rock. They're just curiosities anyway. No one expected that we'd never go back.