Because the Yellow Jersey goes to the overall leader. Currently, Sastre has completed all the stages in the least amount of time. Therefore, he is in yellow.
If you look at the General Classification, you will see sprinters that win stages, hours behind. Oscar Freiere for example. While he has won stages, his overall time is over two hours behind the overall leader.
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If you go here and click on the Tour info, which is on the left side of their banner page, look at Stage 19 results, scan down to the General Classification, you will see what I mean.
Don't confuse stage winners with the overall leader. That's the rider that completes the 2,500 miles or so in the least amount of time.