There could be many reasons. Here are what I think:https://www.bloomberg.com/news/arti...iran-s-400-missiles-request-amid-gulf-tension
Russia rejected Irans S-400 AD system request:
What do you guys make of this?
- With S-400, Iran can theoretically shoot down US, Emirate's and KSA's jets well inside Arabian peninsula. That could increase the tensions even more and increase the chance of a war as a result of miscalculations
- Russia and Iran are allies in some respects and competitors in others. For example, despite what it seems, Russia should be more than happy about recent sanctions on Iran. Reason is, Iran was working on an oil pipeline that would pass through Iraq and Syria and move Iran's crude oil and gas to Mediterranean sea. That would be a big problem for Russia as it would lose its monopolistic position as the sole owner of oil pipe to Europe. So Russia will do its best to maintain the status quo and make sure Iran doesn't become too strong.
- Iran is opening a new naval base in Syria right next to those of Russia. Russia is not happy about this as it would mean more possible attacks from Israel on those areas and possibility of casualties in the crossfire. And again this would help Iran even more with the above pipeline plan. So it may be a show of discontent from Russian side.