Leave twisting to the politicians, Turkey played West hand more than helped Iran in the hour of need, and decision was made immediately after or during Turkey delegation visit (and announcement of cutting down oil imports from Iran). Hence Iran is choosing a friendlier venue, like Iraq or China, whats so hard to understand about it?
Iran hasn't given any economical reason to Turkey for the last three years that Turkey has tried to strenghen its relations with Iran. Why would we choose Iran over US?