Crimea was basically a walk-in. Russia already has a military base there (rented), and just walked out and informed the Ukrainians that they were now in charge. About half of the Ukrainian navy defected and most troops just left, seeing that the "battle" had already been lost. Crimea also had a much higher amount of ethnic Russians and Russian-speaking residents than the rest of Ukraine, due to it's high military value during the Soviet Union. In comparison this invasion has rockets and missiles falling on the capital and areas where almost nobody is ethnically Russian or wants to join Russia.