Why didn't Thanos attack earlier?
Do you think it's because the Ancient One, Ego, and Odin were still alive? Why, or why not?
We're a totally minor target out of the whole galaxy, we don't even have proper interplanetary travel!
He either waited until all the important strategic targets were out of his way, or he wanted to make a particular example of this primitive planet because we're so very overpopulated.