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There are several reasons.
The first problem concerns the design of the lower-power 2" eyepieces - a focal length > 20mm and a wide apparent field of view means some pretty big thick lenses are needed to bend the light to produce such an extreme angular field of view.
Closely related is the large clear aperture needed for the eye lens of the eyepiece, if it is to (a) provide decent eye relief and (b) a wide field. Simple geometry will tell you the eye lens MUST be big.
My Vixen LV 50mm is an extreme example of the above - the eyelens is wider than many finderscopes, and the eyepiece feels like half a housebrick (and almost as big).
Hence all the low power eyepieces from ES, the Naglers, Pentax and even my Vixen ones are B-I-G and heavy.
The smaller sized eyepieces can be smaller sure, for example the TV Naglers.
But - here come the second issue - rebalancing your 'scope if you change eyepieces, and also having to swap between 1.25" and 2" (hence adapter rings). I have tried using eyepiece sets that have widely different weights and on a refractor or Newtonian it can be quite annoying. Hence one of the things I like about my Vixen LVW set - Vixen made the LVW range all weigh about the same, and all except the largest ones have dual 1.25/2" barrels, so the need to use adapters and rebalance the scope is eliminated.
Saving weight on eyepieces isn't a priority. Convenience is.
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