https://www.al-monitor.com/pulse/originals/2018/05/turkey-iran-united-states-can-ankara-bypass-sanctions.html
THE REAL REASON..
US President Donald Trump’s decision May 8 to withdraw from the nuclear pact and reinstate sanctions on Iran comes as a major blow to the Turkish business world.
Turkish investments and trade hadn't really had a chance to benefit much yet from the 2015 agreement that lifted sanctions on Iran. The annual trade volume between Turkey and Iran had fallen steadily from 2012 — when it peaked at $22 billion because of Turkey’s gold exports — through 2016. The decline after 2012 could be attributed to sanctions and political tensions linked to Syria and Iraq. Nevertheless, after the Iran sanctions were lifted, its trade volume with Turkey — which was $9.8 billion in 2015 — slipped to $9.7 billion in 2016. Volume didn't exceed $10.7 billion in 2017.