Why does Montresor wait so long to tell everyone what he has done to Fortunato?"The Cask of Amontillado" by Edgar Allan Poe

In "The Cask of Amontillado," there is a perverse pride that Montesor takes in his revenge and its methodology.  In fact, the motivation for Montesor's narrative is to relate how cleverly he has effected his exact and appropriate revenge upon Fortunato.  This first-person point of view is the lens of Montesor's irrational narrator whose main desire is to describe in minute detail his fulfillment of his family's motto: "Nemo me impune lacessit."

That Montesor tells his tale with the intention of instruction is apparent in his first paragraph as he states the lesson to be developed for his reader: 

A wrong is unredressed when retribution overtakes its redresser.  It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.

Then, in his conclusion, Montesor displays a pride in the success of his plan, the development of his lesson, as he declares that

For the half of a century no mortal has disturbed them.  In pace requiescat!

Thus, he has achieved his objective stated in the first paragraph.