May be they realised their carriers, warships and military bases in the region within the 1000 km of the Iranian missile range are not indestructible.
The Iranian leadership are willing to take out as many US military hardwares and infrastructure with them. They know they can't win a war with the US, but they are willing to give the bully such a bloody nose that Uncle Sam will think twice before attacking Iran with impunity again.
In the meantime, it is in both Russia and China's interest to safeguard Iran's survival, integrity and a sovereignty.
Iran's defence must be bolstered.
It won't be a bad idea for China to test their defensive missiles in Iran against the US aggression.
The USA is using economic pressure to destroy Iran, and other sovereign states that dare challenge it's hegemony status.
Both Russia and China are been discriminated, with their economy being attacked by the USA.
China must refute the USA's illegal international oil sanction imposed on Iran. This was trumped up unilaterally by the USA, and they are forcing every country in the world to follow suit by various arm twisting means.
The USA is an evil regime that must be opposed by all self determined states.
In power, Trump's diseased mind ordered many international treaties signed by the USA to be torn and discarded. Iran's joint agreement on the de-nuclear research was unilaterally scrapped by the USA, and their renew call for Iranian oil and gas sanctions.The EU and Japanese have already bow to the US threats and stopped their shipping of Iranian crude.
Naturally the country benefited most on the renew Iranian oil sanction is the USA. The US oil and gas fracking industry is now pumping the extra billions into the US Treasury.
China must resist Trump's evil motives, his bullying tactics to "make America great" regardless of international laws and norm.
Many Chinese multinational oil importers with the dollar account have already terminated business with Iran, no doubt on the advices from their bankers and investors.
China the country have the power and leeway to kickass the US bullying. China should continue importing Iranian oil, on the theme of oil for yuan.
India is also ignoring the US threat tn the Iranian oil import.
With the US unilaterally scrapping the multinational joint agreement on the Iranian "denuking for economic freedom", Iran is now being pushed back to their starving phase.
Iran declared it will renew its nuclear research soon, a self preservation move.
China and Russia must use the Veto rights to stop any attempt by the US and their allies to forcing UN sanctions on Iran.
It was this UN nativity of Russia and China that brought the destruction to both Iraq and Libya by the Western military force. Don't fall for the UN entrapment again!
China and Russia can put pressure on the US military, deterring their warpath, their wanting destruction of Iran by assisting Iran on the movement of the US military.
It is of no secret that both China and Russia have the real data on the movement of every US military shipping, boomers and planes.