Availability is probably the answer.

I have some Trinitite. It's easy to obtain. You can buy it in rock shops, especially in the Southwest. Back in the day, when they opened the Trinity site to the public a couple of days a year, there was no prohibition on collecting Trinitite. That's why rock shops have it.

They still open the site to the public one or two days a year but they won't let you collect Trinitite any more. You can walk on it but don't try to put it in your pocket or a person with a gun will pull you aside for a chat.

https://www.nps.gov/thingstodo/alamogordo-visit-the-trinity-...

There's not much public access to the sites in Nevada; at least until the DOE resumes their bus tours and they're even more hard over about not letting you pick up anything.

https://nnss.gov/community/monthly-community-public-tours/