I think that Harriet is less regretted and the intimacy between her and Emma sinking is due to two things: Emma coming to her senses and the awkwardness of Harriet aspiring to marry the man Emma is marrying.

The way I see it, Emma finally realizes that what she was playing at with Harriet was a monstrous intrusion on a life that would have been happier had Emma never meddled in it. Had she never had anything more than a calm good will toward Harriet, Mr. and Mrs. Robert Martin would have had an additional year or so of happiness: Emma nearly ruined it altogether. Awkward!

Also, I think Emma finally concludes that Mr. Knightley was right all along: she and Harriet were simply not well suited as intimates. If you look back at what he said in chapter 5, you see that it pretty much had a grip on what was wrong from the beginning.