Iran is on the southern coast of the Caspian Sea, with the Volga flowing into it from the north. The Volga River is large, navigable and has a vast basin feeding into it that goes deep into Russia. The surrounding areas didn't have as easy of access to the larger, richer and more advanced civilizations far to their south.
What were the effects of this? Iran had great and advanced civilizations going back to Elam and likely earlier.
Interesting question. To be honest, I don't have a good answer. There are some things I'd like to add though:
The Rus' were known to raid the Iranian Caspian coast. I know it's Wikipedia but this article is surprisingly good.
Volga Bulgaria, a state that existed in what is today part of the Russian Republics of Tatarstan and Chuvashia, converted to Islam under the influence of Muslims from the south. If I had to guess, these were almost certainly Iranian Muslims from Samanid territory, who were also responsible for the conversion of Turkic tribes in Central Asia. Volga Bulgaria was destroyed by the Mongols in the 1240s. However, Tatarstan and Turks in general still follow the Hanafi madhhab propagated by the Samanid dynasty.
Astrakhan, located where the Volga enters the Caspian Sea, was historically an important stop on the pilgrimage route to Mecca. When the Russians conquered the Astrakhan Khanate, the Ottomans and their Crimean vassal declared war on the Russians - they failed to capture the city but the Russians promised Muslims access through Astrakhan. (Also, I have this illustration of 18th century Astrakhan by some European merchant calling it a "Persian city" but can't find it right now.)