They did a cost benefit analysis and decided there were few risks to taking Crimea now. But there is a cost. Crimea will cost money to the Russians for pensions and infrastructure for very little benefit to the Russian people. Empires do cost. And the Russians will have to spend even more on defense now because everyone else in their area of the world will be doing the same.
Actually Crimea has a lot of valuable gas and oil beneath its sea shelf waiting to be exploited - and the potential is considerable. The profits from its development will more than offset any potential liabilities stemming from its acquisition for Russia. This is made clear by the sanctions America slapped yesterday on the Crimean state energy company Chernmorneftegaz to discourage its transfer to Russia’s gas behemoth Gazprom.