why does the sexton admit the narrator into the churchyard?

The sexton was acquainted with the narrator, that's why he allowed him in.

"I got admittance into the churchyard by being acquainted with the sexton who attended; who, though he did not refuse me at all, yet earnestly persuaded me not to go, telling me very seriously (for he was a good, religious, and sensible man) that it was indeed their business and duty to venture, and to run all hazards, and that in it they might hope to be preserved; but that I had no apparent call to it but my own curiosity, which, he said, he believed I would not pretend was sufficient to justify my running that hazard."