QuficMan wrote:hmm.. good point!
But still, why not the second run. Someone said a shortage of torps, and I think that might explain it. But still, wouldn't they be able to find a few more torps, after all, its a high profile mission to say the least!
Some of you people think to much. 
First why would they do a trench run, where a normal run from the top of the thermal exhaust pipe would have been better?
Easier to line up, plus you wouldn't be boxed into a trench and killed most likely.
Now if that wasn't possible it meant you had to be lined up for the trench run. And seeing lining up took time, they only had one run each. Because getting back to the start of the run would either take to long or would be to dangerous.
I do believe they had more then 1 set of torpedoes. Check the wiki link.
http://en.wikipedia.org/wiki/X-wing