That's what they want you to believe.
Same reason they haven't had one in Dubai, Qatar, Tajikestan, Kuwait, Azarbaijan, Greece...etc. There's no money being paid for blowing up Iranian shopping centers and mosques.
The reasons BT jaan are a lot dirtier. One of the reasons is that the IR is a terrorist regime itself and terrorists know each other. The IR has a huge terrorist network around the globe and specially in the region. IR is factually hosting the heads of Al Qaida and until 5 years ago even the sons of Ben Laden while they were hunted globally even in Saudi Arabia. The IR is on record issuing passports and letting sunni terrorists of Al Qaida and even ISIS members to pass the iranian territory without any issues from Afghanistan to Iraq. In return they have promised to keep their hands off iranian cities. ISIS has all the tools to do a lot of damage to iranian cities and even to IR officials because they dont fight to survive, they fight to kill and die. They can easily blow up a vezarat khoone in Iran, they can easily blow themselves up in squares, in cinemas, in gatherrings... They have their people in Iran, in Balouchestan, in Tehran, in khouzestan, in Kordestan, Kermanshah, Azarbaijan....but they need IR in their fights in Iraq and Syria. ISIS wants to fight and the IR wants the fights in Iraq and Syria to extend too. IR, ISIS and Al Qaida are enemies who have found some strategical common ground so while there are fights between them in Syria and Iraq, there is peace with the same elements in Iran. In Yemen as example the IR has used Al Qaida fighters as exchange material for the shiite terrorists who got into the hands of Al Qaida in Yemen.
These motherfuckers are terrorists by nature and terrorists always find ways to keep each other in check.