system
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human
[Star Wars]A New Hope Medal Ceremony... (not about Chewie and Wedge)

The day after the Battle of Yavin and the Destruction of the First Death Star, Luke Skywalker and Han Solo attend a ceremony in their honor. In attendance for the ceremony are no fewer then 90 pilots.  Two groups of 40 and two groups of five. Where were all of these pilots when the death star was being attacked? 

-Red Squadron sent 12 X-Wing starfighters of which only two Luke Skywalker and Wedge Antilles surviving. 

-Gold Squadron sent 7 Y-Wings of which only one returned Keyan Farlander. 

-Blue Squadron sent 6 X-Wings and to my knowledge suffered between 0-2 casualties. 

-Green Squadron sent I believe 8 X-Wings and their casualties were unknown  

Even Excepting that neither Green nor Blue Squadron lost a single man, and allowing for Antilles and Farlander to have been part of the ceremony audience as well, we are left with 74 pilots who did not participate in the battle. Why not? I get that you don't want to send every pilot you have and risk losing them all. But only sending 1/4 of them seems a bit risky as well given the magnitude of the task at hand. 

So my question is, why? Why didn't they send more pilots? I know that at the time there was no an abundance of X-Wing fighters in the alliance but there had to be Y-Wings and other craft. In a go for broke everything or nothing attack you would think you would want to send all of the help you could.
gpt
Lack of combat worthy craft. This was the early days of the rebellion. They literally sent all the fighters into the battle that they reasonably could.

Sure, the rebels had some atmospheric craft, some fighters that were not currently flight worthy, and some heavier craft (combat modified cruisers and cargo ships, mostly), but none of those were compatible with the all starfighter trench run strategy.

Here's the real thing - in the early days, the rebellion was full of spirited revolutionaries, ready to jump in and strike out against the Empire. Many of them had experience with planetary security, and some had even gone through the Imperial flight school. The problem is, a bunch of good pilots come and join the rebellion. They didn't necessarily have access to starfighters, and those that did couldn't necessarily bring them with when they joined up. As such, the rebellion ended up with secondary and even tertiary crews for every fighter they had, not having enough ships for all their pilots.

Sure, later on they got better donors, captured more materiel, and became the New Republic, with proper taxation and acquisition powers. Then, they were able to supply fighters for all appropriate pilots. But early on, at Yavin? All the pilots at that medal ceremony were on the ready to go fly, so they all were able to attend, but they just didn't have anything that would let them join the battle.