That may be because not many roadsters were sold and the rarity makes them more desirable. The newer models sell in larger numbers and (i assume) will fall faster in the used market.

It's possible. The P85, for instance seems to have depreciated more rapidly. But that might be because Tesla keeps pushing the line out on their top of the line model.

By comparison, though, the 2011 Mercedes SL500 cost around $105,000 and is currently worth (depending on mileage) right around $50k. That's more typical depreciation for high end sports cars.