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Well, how much field of view do you want, and how well-corrected do you want it to be?
Want a widefield with superb edge correction? Then more elements will be added to correct residual aberrations. Want even MORE field (like 100 degrees, 110 degrees, or 120 degrees)? Then, add even more lenses.
Want usable eye relief? Likely means the eyepiece will have a negative lens ensemble at the bottom and larger lenses up on top (because most eyepieces today resemble long focal length designs with negative lens ensembles added). 100 degrees? Maybe 9 elements. 120 degrees? 12 elements.
What makes that possible is incredibly expensive and elaborate coatings on the lenses. Otherwise, that many elements would be too dark.
So, make the lenses bigger, add more of them, scale the barrels up to hold those lenses, and what do you have? Heavy eyepieces.
I saw a 6" diameter eyepiece used in the 200" Hale telescope for testing. It was 18 lbs!
Sort of makes all our little 2 lb eyepieces seem light, eh?
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