(1) Iran has always adopted and implemented anti-Western (American) policies since the Islamic separation. At this point, it is almost consistent with Russia (including the former Soviet era). Therefore, it is an inevitable choice for Russia to protect Iran.
(2) Since the disintegration of the former Soviet Union, the Russian economy has gone through a difficult period. During this period, "assistance" and "bilateral cooperation" to Iran are one of the important driving forces to maintain Russia's economic development. If there is a positive change of power in Iran under the attack of the United States, it will undoubtedly be a radical change for Russia's economy that has just recovered.
(3) From the geographical point of view, Iran and Syria are the last two pillars of Russia to support its "southern line" in the Middle East, and once they "have an accident" again, the Mediterranean and Central Asia will be completely controlled by the United States, and Russia will fall into an increasingly unfavorable passive position.