Because the US and Israel keep threatening to bomb the **** out of Iran (just last week for example). The UK and US have been threatening and acting against the Iranian people for over 100 years, so the Iranians have a real good reason to not trust the US. Keep in mind that the plant is under 24/7/365 monitoring by IAEA and is fully in compliance with NPT. You just don't like the fact the Iranians don't trust you (for very valid reasons).
Iran's oil exports to South Korea quadruple to 400,000 barrels a day after sanctions lift (Reuters)
from the article: Report: Iran begins uranium enrichment at underground site