1. The Crimea has historically been Russian ethnic.
2. Joseph Stalin reassigned it to the Ukraine over their wishes on the grounds the USSR was all one thing so it didn't matter anyway.
3. The overwhelming majority of Crimeans supported the reallocation back to Russia. Sort of like if modern Alaska was in Russian control and voted to join the US instead. Only makes sense.
4. The Deep State controlled media opportunistically abused it as a chance to bash Putin for not playing along with their Rothschild-owned central bank model for all governments in the world.
Think what you want about Putin, but this one's not rocket science.