Proust’s Combray pages narrate years of habitual childhood in the iterative, then give a single evening dozens of pages. Why does the shift itself carry the emotion?

D. Because the change of frequency marks that evening as the one that broke the pattern

Because the iterative has established a rhythm of what always happened, so the moment a single evening is narrated in its own right, the form announces that this one was different before a word says so. The technique is not weaker or stronger, the meaning is in the change, which is why neither mode could produce the effect alone.