What difference would it make if it were a peach in an orchard? What does the apple is the garden represent?
I would consider that Blake chose to use the idea of the apple tree to contextualise the poem as a religious sermon - a former title of the text was Christian Forbearance' The apple symbol fits better with the idea of the Garden of Eden as the narrator is tempted into revenge which he cultivates carefully -
And I watered it in fears
Night & morning with my tears,
And I sunnéd it with smiles
And with soft deceitful wiles.
An earlier e-notes answer suggests that the apple represents anger, and certainly its growth and brightness suggest that an emotion such as anger develops with the attention it is given by the narrator.
Another point would be that apples were very much more a part of the English garden in Blake's time, whereas the general climate in England (with the exception of sheltered positions in the sunnier southeast regions) was not really conducive to the peach.