WHBM
Military chain of commands is so much shorter (ok, generally speaking...) so it's more "fix this YESTERDAY and that's an order!" (ok, overemphazising on purpose).
Civil aviation has all kinds of "red tape" running all across the world and with an Ethiopian plane on a UK tarmac with a US built airframe, insured by Someone and on top of that a major
PR stunt for Boeing, of course it takes time no matter how well prepared the engineers might be.
Once the work started it seems they did pretty well - remember we don't know how extensive it was on the inside. I do believe that composite materials are pretty well understood IF Boeing chooses to use the military knowledge people in the civilian process. I am not at all familiar with Boeing's corporate structure but there are occasions where military and civilian engineers are completely separate within the same company (you know, secret stuff, classified stuff, not allowed to share because of the safety of the nation etc), leading to peculiar gaps in information exchange. This will of course hamper the development of products, mainly on the civilian side because that side is the one that really has to consider price tags.
Now, as for how they actually fixed it, I am a curious beast. Any gossip, loose rumours, uneducated guesses, anything besides the post in Seattle Times? ;-D