Here's a little test for somebody who wants to patent a product and make a small fortune. Seriously, take ten seconds and do this:
While standing up, lightly grasp your crank-and-balls with one hand. Hold them together in one hand like a protective cup does. Now, with your hand in the same place with the same light grip, kneel into a catcher's position. What happens to your crank-and-balls? When you kneel, they start to come out from underneath the bottom of your hand. A cup is shaped very similar to how your hand is when you "cup" your junk together. Cups are put in while standing up. When a catcher kneels, the cup starts to point out - the bottom of it raises up and points towards the pitcher. This allows your balls (we are good with not using medical terms here, right?) to be exposed, especially from the bottom. This is why you see the most damage done to catchers on foul balls that go directly into the dirt and then bounce back up from underneath the catcher's junk region.
It doesn't matter if you put the cup in a built-in slot in sliding shorts, wear a separate jockstrap for it, etc. The same phenomenon happens. If somebody would create a device that stays attached and doesn't raise up when a catcher or athlete kneels, they would be able to make a small fortune licensing it to big manufacturers/distributors.