It is probably a withdrawal in exchange for the recognition of the autonomy of the Crimea. Of course, the EU and USA's sanctions etc. have also been effective in this. After all, Putin was not the only person who ruled Russia, the EU and the USA could have put a big stick in the wheels of the oligarchs.
Good news for Ukraine. If they can somehow throw themselves into nato or eu without a war, those who want to leave ukraine in time and those who leave will regret it. Crimea is one of them, the referendum held there was of course dubious, but the pro-Russians were in the majority. This picture may come to a very different point after 10-15 years with the development of Ukraine. Of course, in the meantime, if Russia removes the Crimea from its autonomy status and includes it in Russia, they will not have such a chance. this also applies to some regions in the east of ukraine.