difference between "has to start" and "not started"

1. I'm waiting because the leader has to start the event.
2. I'm waiting because the leader has not started the event.

What is the meaning of above two sentences?

There is no particularly substantive difference between the two sentences; they both convey the idea that you are waiting because the event has yet to be started by the leader.

The only difference is the choice of words - "has not started" describes the situation by discussing the lack of an action having occurred, whereas "has to start" describes the situation by discussing the anticipation of an action. If ever a choice had to be made between the two in a writing piece, that choice would be determined entirely by the context and the writer's choice, such as in the following example:

... in summary, because the process is so complex, it's been designed so that the leader's approval is required for every major step of the process. That's why I can't start working on that project - because this event has to be triggered first, and the leader's the only one that can do that, but he isn't here yet. I'm waiting because the leader has not started the event.

vs.

... in summary, because the process is so complex, it's been designed so that the leader's approval is required for every major step of the process. That's why I can't start working on that project - because this event has to be triggered first, and the leader's the only one that can do that, but he isn't here yet. I'm waiting because the leader has to start the event.