I think you pretty much answered your own question as you were pondering it.
Either the Lorien Elves stopped trying to keep up with the ever-morphing language of their mortal friends (whom they rarely saw anyway), or they figured it would be a waste of effort from the get-go to learn a tongue they would probably seldom use.
Although, if I had that much time on my hands, I would learn a new language just to keep from going insane with boredom.
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The Dwarf breathes so loud I could've shot him in the dark, drunk, blindfolded and hanging upside down from a tree.
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