It's because he's fun to use and also quite viable, imo. You have great set up for a potential gank when you master box placements and predictions. Also you have great ability to deny enemy ganks as Shaco support, again if you master box placements and predictions.

Again, he's pretty fun to play building AP. I know that I can't burst everyone and can't carry games through killing damage dealers or through split-pushing, but I know I can change clash situations through boxes and baiting with my clone. AP Shaco is only good at clean ups and mind games. He can't carry alone unlike the near one shot full AD Shaco, Ap Shaco needs a responsive team, especially when baiting enemies so you can pick up a kill together.

When laning don't spam your 3rd cause you'll run out of mana quickly even with manaflow band. Always have a box on where you and your carry are gonna back when ganked or all in'd by your enemy. Be mindful of the cd of your enemies' sweeping lens so you won't waste mana and box placement options when your boxes gets proc'd by sweeping. Don't be the one to back first, you are a support not a carry. Don't be afraid to take damage as long as you know you'd survive, that's a good baiting potential. Always keep an eye on your cooldowns especially your 1st and 2nd. For more tricks to gain just watch Pink Wards montages, it sure helped me improve my box placements and decision making as Shaco as a whole.