Upload & Sell: On
Answer to the original question:
Just because it is the best camera made by Nikon. The D4 really does add anything spectacular. In fact the D4 is a serious step backwards in terms of compatibility. The D4 is not only much more expensive, but you have to add batteries, new incompatible card readers and ridiculously expensive memory cards (XQD), making the total package between $500 and $1000 even more expensive.
For my kind of 'allround' photography, I regret having bought a D4, but the D3s was not available anymore when I made the choice.
Guy, if you regret buying the D4, I'm sure you won't have a problem finding a D3s owner who's willing to trade for your D4 Actually, I wanted a D3s initially. But for about $1K more, I realized that I should just get a used D4 since this is a body that I'll keep for a long time. I went for the newer tech and I am so glad I did. You can get genuine En-El18's on ebay for around $60 if you look hard. XQD is awesome, but the D4 can still take CF's and those are cheap as dirt.
To answer the OP, I think it's because the D3s is (was) a flagship body, and flagship bodies usually hold their value really well. Just look at the D3, it's still selling used for $2K, and it's almost 6 years old! I think we're at the point where sensor tech isn't improving by leaps and bounds unlike 10 years ago. The D3 and D3s will continue to perform for years to come.. and that is why I didn't hesitate to buy a D4 because I know that it will keep me happy for a LONG time.