Rivendell is so sprawling that it's hard to find a convenient place for RP; same with Lothlorien (made worse by the rep issue). I did see RP in Mirkwood when the zone was fairly new, but it's something of a cul-de-sac these days so I would expect there to not be the population for it.
When you add all these factors to the fact that Bree and/or Michel Delving are both known to be popular RP spots, I suspect it's just naturally easier for RP to gravitate to there.
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