... not necessarily because Harriet is separated from Emma by class distinction.

(ch. 55)
Robert Martin is introduced at Hartfield, and appreciated by Emma.

Emma accompanies Harriet to the church for Harriet's wedding -- and participates?

"Harriet, necessarily drawn away by her engagements with the Martins, was less and less at Hartfield; which was not to be regretted. The intimacy between her and Emma must sink; their friendship must change into a calmer sort of goodwill; and, fortunately, what ought to be, and must be, seemed already beginning, and in the most gradual, natural manner. "

I like to think of it as being the natural consequence of Harriet's integrating into the family of her in-laws, and the distance (that I, at least, infer) between Hartfield and Abbey Mill Farm.

I really like the idea that Emma will now be able to "notice" the second-rate of the neighborhood, along with Mr. Knightley and the nouveaux gentry like the Westons and the Coles.