I saw the movie about a million times, read the book at least three times and never quite understood the trench run attack until today when I rewatched the scene. I was especially confused about why we see only two X-Wings at the beginning of Luke's run in a short cutscene. But now I think I understand.
In the first two runs, Dutch and Dave went in with their wingmen close behind. This gave them additional chances for hitting the target, but the wingmen also operated as "human shields" against squadrons from behind. They were focusing mostly on hitting the target for sure, and relied on their shields as protection. But that made them easy prey for the much faster TIE/A as Vader had plenty of time to close up and destroy them.
Luke chose a different approach. They went in at full throttle "Just like Beggar's Canyon back home!", but more importantly, he went further ahead. With this Vader had to choose: Attack the lead ship and risk being fired upon form behind by Biggs and Wedge, or deal with the defenders first, risking that he won't be able to catch up with the lead ship in time. In the end it went down to the Rebels not knowing what Vader and his custom TIE are capable of, while the Empire knew the exact capabilities of the X-Wing. At full speed Biggs and Wedge had very little energy left to their shields making them easy prey. Vader also knew that a damaged X-Wing won't be a threat because it won't be able to keep up. The TIEs are faster in straight line speed than the X-Wing, and a damaged one would quickly fall behind, out of firing range. He damaged Wedge, destroyed Biggs and after disposing the wingmen he still had enough time to catch up with Luke.
I know, I'm such a nerd. I love Star Wars