The reason to play Juniper is because it gives you a solid seven card hand that you can play, which is massive card advantage, while Caitlin gives you one less card than you had. It's minus cards that you're able to play. If your hand is atrocious, like three Professor Juniper and four Double Colorless, then yes, I'd wish I had a Caitlin. However, there is always a situation in which one card is better than another, and Juniper is more useful in the vast, vast majority of situations than Cailtin is. Even in the above situation, I'd rather have the far better Cheren to draw me three cards until I grab the N to send the hideous hand back into the deck.
You get the best starting hand imaginable. You play every single card in your hand, search your deck for awesome basics, attach the energy you need, set up your discard pile, and now you have one supporter in your hand. Do you want a Caitlin, or do you want Professor Juniper?
Your hand has junk in it, none of these cards help you in any way, and off of your last turn supporter, you have a four card hand and one supporter. Do you want Caitlin, or do you want Juniper?
If everyone plays N, and you're somehow ahead with your Caitlin deck, and the opponent N's you to three cards, and you have a supporter in your hand, would you want a Caitlin or Juniper?
Here's why people use N:
You get the best starting hand imaginable. You play every single card in your hand, search your deck for awesome basics, attach the energy you need, set up your discard pile, and now you have one supporter in your hand. Do you want a Caitlin, or do you want N?
Your hand has junk in it, none of these cards help you in any way, and off of your last turn supporter, you have a four card hand and one supporter. Do you want Caitlin, or do you want N?
You're behind with your Caitlin deck, and the opponent has an eight card hand and three prizes left. You have five prizes left and an okay hand and one supporter. Do you want to play Caitlin, or do you want to play N?
So, yes. Caitlin is pretty weak when your opponent can do 100+ damage on turn 2 and you're still doing weak hand refresh digging through your deck two or three cards at a time, effectively netting +0 cards by the end of your turn because you get minus card advantage with it. Shauna is a good card for Speed Lugia EX in place of some N, but even then, it's a big niche. Not Caitlin, though. You trade them to your friend named Caitlin for the sake of novelty.