My personal feelings aside on that (I, for one, loved Kenneth Brannagh's Hamlet, because it stayed very true to the play despite being set in 19th C. Denmark...other offerings, like the modern R&J, or "O," based on Othello, I didn't even bother with...), I believe the perception is one of memory.
We Trek and BSG fans remember sitting in front of the TV and watching the original shows. They remain fresh in our minds and hearts, and we've invested part of ourselves into these shows. For Shakespeare, none of us were around back then to see his original presentations, so all we have are the interpretations of modern directors, producers and actors. Some work (like pretty much all of Brannagh's Shakespearean movies), and some don't (the modern R&J and "O").
Another example is Homer's Odyssey. I have seen two recent works on the story -- both very different (the made-for-TV Odyssey mini-series and "O, Brother, Where Art Thou"). For me, both worked very well (and for those of you who have not seen "O, Brother..." it is worth the time!
). Yet, for Star Trek, I have a much greater scrutiny, because of my years of invested time and energy on the original series and its spin-offs. As such, I can be more critical of each new series or movie.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz