Because delivering bad news is harsh, and you may end up hurting the person's feelings. Ultimately, business doesn't care about feelings.
You have to deliver the message, but you also need this individual to deliver results. So framing the conversation in a manner in which they actually understand and positively receive your message is very important.
I'm going to give you the verbiage that is going to allow you to confront your sales leader without being pulled into their web of lies as to why they didn't hit their targets. When you sit them down, let them know you're going to give them an opportunity to speak their part. But you need to communicate to them what you see first. Then start with the facts. Let them know you see that the weekly targets aren't being hit, the monthly targets aren't being hit, and you feel that the quarterly target will follow suit. It would be irresponsible of me to allow this madness to continue and not say something.
The reason why you want to start with this position is because you want to curb their perspective. People convince themselves of all sorts of drama. So allowing them to speak their piece. Usually, CEOs have a very difficult time having this conversation with their sales leader, because if they open up a conversation, the sales leader is able to muddy the waters with what they should actually be measuring, what the actual challenge is versus what the actual results are, and even make the missed targets a foregone conclusion. You need to move past it and actually look to solve the problem.
Explain to them what needs to be true by the end of the quarter. We need to be at X, Y and Z revenue or whatever metric it is that we're measuring. Then give them an opportunity to state their position and ask them what would it look like for you? What would you like to accomplish? What can you accomplish? And what are you committing to if done properly?
This is going to prevent the conversation from going down the road of talking about leads, cheaper pricing, or something to do with the competition.