The dumb little thief used most of Russia's massive AA missile stockpile to commit war crimes (bombard civilians) after they ran low of their actual missiles (because they bombarded the civilians so much). They simply haven't got that many left. Also, Ukrainians got really good at finding and destroying the launchers. They have few missiles and fewer launchers still left. The few that are left are in use in the warzone, not 600km behind it.

Source: armchair general with zero actual military knowledge