What with all the crap flying around up there. Go check out the ESA pics of LEO objects to get an idea of just how crowded the skies over earth really are. Here’s a pic of the current number of satellites around the planet from the point of view of the equator:
From that point of view it looks like there’s no room for space shuttles and probes to make it into space. Obviously there’s still enough room that it’s not too much trouble, but it was still surprising to see just how much crap we’ve got up there.