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Because Easter (as with many "Christian" holidays) actually originates from an ancient pagan holiday. It was originally a celebration of the goddess Ostara, a goddess of fertility. Her sacred animal was a rabbit, and eggs are a pretty common symbol of fertility as well. Somewhere from all of this came the idea of an egg laying rabbit, which ended up associated with Easter even until today.
So basically: pagans. Same place we got Christmas trees and mistletoe.2