Because when you get over/get used to the bitterness, theres a lot of flavor in coffee. You just need your senses to either acclimate to it, or you need to get older. As you age, you lose sensitivity in some taste receptors which makes formerly overpowering flavors less offensive.

Same thing for tea, and cheese, and most things kids hate but adults love. Thats why youre supposed to try things again every 5-10 years.