As with any fruit, the longer it stays on the mother plant, the sweeter the flavor. This is due to the development of the sugars within the fruit. Commercial growers just don't have the time to let the fruit sit and develop. They have to pick and gas them and get them on the truck to deliver to the market ASAP. That's why oranges at the market aren't orange and apples are hard and bitter.
It's sad that as a society, we have forgotten the growing traditions. The market is quicker and easier and more plasticky. The market is bad for the environment and your wallet!