It's the classic case of not choosing your bedfellow wisely, especially for the West. Although what started the Maidan Revolt is something else, by the time it reached its first apex things have changed with the involvement of Right Sector and the like, and they hijacked the revolt plus adding their own agenda...the US and EU ignored that development, thought anything that'd weaken Moscow's stance is good, totally disregarded those Bandera freaks will do nothing less than giving Moscow the rallying cry it so badly needed.
As we can see in the subsequent events, not even the change of government at Kiev incited uprising in the Eastern Ukraine (where Russians formed an ethnic majority), but when the "new" parliament bulldozed that law through, Moscow now got all the local support it needed to muscle in. Sure enough, "rebels" or "local militia men" who are that well armed, disciplined and well trained are either front-line graded regulars or even special forces; mass defections by Ukraine forces stationed in Crimean region further shown how little the "new" government in Kiev got when comes to loyalty from the regular troops, which they're now dealing with the issue by drafting Right Sector thugs into newly formed militia outfits.
Then when comes to response, the Western Powers simply has neither the resources to totally replace the "Russian factor" in Ukraine's economy, nor the stomach to really challenge Russia, a nuclear-armed superpower (to all those hippies out there, that's the prove of nukes keep the world peace!); when comes to sanctions, the biggest Russia-EU trade is LPG (try to live without heating when General Frost comes knocking!) which EU couldn't hope to find alternatives in short term, the US pledged to meet the demand with shale gas but we all know "fracking" is a very problematic method to say the least; the UK ain't prepare to have its financial industry get hurt in this either...
And there's China: with China's economy still going strong and sourcing energy cleaner than coal, Russia really have no worries when comes to fossil fuel exports. Pretty much if the West really want to make Russia back down they should've woo China to join in, but when SecDef Hagel badmouthed China so much during his recent Asia tour, you can tell they never consider getting China onto their side. Thus you can see Beijing pretty much let Moscow do their thing, while the Foreign Ministry issued a few statements which pretty much equivalent to have said nothing at all.
Thus far, it can be said that the Western powers handed too much good cards to Russia.