In The Scarlet Letter, why doesn’t Dimmesdale confess until the end?

Dimmesdale's internal conflict has always been "confessing versus not confessing". As early as chapter 11, "The Interior of a Heart," we learn that, while Dimmesdale tries very hard to come to terms with his cowardice, he has also tried to fit his truth, somehow. The problem is that the villagers are blind followers, as such, they will follow their extremely beloved Dimmesdale regardless of what he says to them. They have adopted him as their "young divine." They also treat him as some sort of preternatural being. Therefore, no matter how subtlety Dimmesdale tries to accommodate the fact that it was he who impregnated the pariah of the Puritans, Hester Prynne, his comments will fall on deaf ears.

He had told his hearers that he was altogether vile, a viler companion of the vilest, the worst of sinners, an abomination, a thing of unimaginable iniquity; [...]Could there be plainer speech than this? [...] Not so, indeed! They heard it all, and did but reverence him the more. [...]

Fast forward to chapter XXIII, where we find Dimmesdale again willing to disclose his truth after his most important sermon on Election Day. This time, he does it close to a time when he knows that his end is near. He knew he was dying, and that after the confession, he will go meet his maker, and the world will continue as usual. This was a low blow for Hester, since Dimmesdale even tells her that they will not meet in eternity; that each will meet their fate as God sees fit.

Other reasons to confess at this point include Dimmesdale's inner turmoil, which was consuming him from the inside out. He had carved a letter A on his chest, endured self-mutilation, gone depressed and emaciated, and his brain was in a consistent state of anxiety. Sooner or later, he needed to let go of this burden and move on for good.

By bringing me hither, to die this death of triumphant ignominy before the people! Had either of these agonies been wanting, I had been lost for ever! Praised be his name! His will be done! Farewell!

Therefore, Dimmesdale wanted to confess, but he decided to wait until this poignant moment in the day of the villagers to get his point across...and die quickly to avoid the earthly consequences that will come as a result of losing the place that he has acquired at the top of the pedestal of the community.