Multiple reasons. 1st Russia exports a lot of natural gas to EU. Majority of this goes through Ukraine which makes it a critical region. Ukraine's importance in relation to the Russian Gas is also one reason why some actors are wary about Nord Stream expansion.
Second is the military base in Crimea which was already covered.
Third was the cabal plan of escalation between nato & rus. If you remember the rhetoric was pretty tense before 2016 elections and was heating up.