Jeff Findley writes:

The shuttle's tiles did show some "streaking" after several flights, but
it wasn't as prominent as what is seen on Dragon.

The bottom of Dragon is protected by an ablative heat shield which
deposited some of its soot higher up on the craft. If you've ever seen
a "flown" Apollo capsule in a museum, you'll see they look quite similar
to a flown Dragon. No doubt SpaceX will want to clean and paint the
upper surface (or simply replace the outer panels with new ones) before
reusing a Dragon so the soot does not change the thermal properties of
the craft in orbit. That and it looks better to launch a shiny
spacecraft than a dirty one. :-)

Another thing to consider ist just scale. The "streaking" on the shuttle
looks much less prominent compared to the size of the craft. Dragon is
much smaller though.

Look at this picture of a landed shuttle:

http://ww2.hdnux.com/photos/03/42/63.../3/628x471.jpg

It looks not much better than Dragon if you look closely. And then: some
photos of the current Dragon were from a side that was very sooted --
look at this photo, it doesn't look half as bad
http://pic.twitter.com/OqF7ghXZ


Jochem

--
"A designer knows he has arrived at perfection not when there is no
longer anything to add, but when there is no longer anything to take away."
- Antoine de Saint-Exupery