Why won't the lucky coin drop after 69 pirate invasions?
My personal experience was the same as yours, I did many pirate invasions to get just one lucky coin, AND just one discount card. Ironically for me, they both dropped in the same invasion. As @Waterseas pointed out, it is a 1/4000 chance, and yes you can be just that unlucky thanks to the mechanics of RANDOM(). On the positive side, by now you should be able to fully outfit a king's castle in golden furniture!