This is why:
U.N. nuclear chief: Alleged weapons testing site was probably sanitized by Iran
The United Nations’ chief nuclear official urged Iran on Thursday to allow inspection of a military base where Iranian scientists are suspected of conducting secret nuclear-weapons research, although he acknowledged that any traces of illicit activity have probably been removed.
International Atomic Energy Agency Director General Yukiya Amano said the nuclear watchdog would try again next week to visit the Parchin military base, a sprawling complex where Iran is thought to have conducted tests on high-precision explosives used to detonate a nuclear bomb.
Iran’s quest to possess nuclear technology: Iran said it has made advances in nuclear technology, citing new uranium enrichment centrifuges and domestically made reactor fuel.
Iran has repeatedly refused to let IAEA inspectors visit the base, on the outskirts of Tehran. Instead, in the months since the agency requested access, satellite photos have revealed what appears to be extensive cleanup work around the building where tests are alleged to have occurred.
“We are concerned that our capacity to verify would have been severely undermined,” Amano told a gathering of the Council on Foreign Relations in Washington. He noted Iran’s “extensive” cleanup effort at the site, which has included demolishing buildings and stripping away topsoil.
“We cannot say for sure that we would be able find something,” Amano said.
That's one hell of a good reason.