"Stormtrooper aim didn't outright suck until Obi-Wan "died". Obi-Wan gave himself up to the force and used Battle Meditation to keep the stormtroopers from hitting the main cast for the rest of the series. This is what he meant by 'If you strike me down, I'll become more powerful than you can possibly imagine.' I thought this was obvious?"
(discounting the prequels cause)
A. why wouldn't you?
B. they aren't "Storm troopers"
In New Hope, the Stormtroopers expertly destroy a Jawa sandcrawler, they massacre Uncle Owen and Aunt Beru, and apparently these sand people. The second Obi-Wan shows up, their aim starts to suck.
BATTLE MEDITATION - the GREATEST Force Power - explained:
Battle Meditation (discussed/featured in Knights of the Old Republic) is easily the greatest force power imaginable. It provides complete and total sync with all engaged forces during a battle, enabling a "force general" to command the assault mentally. It's HEAVILY implied that Palpatine was using this during the Endor Assault prior to getting tossed down the reactor core.
Uhhh... decade old MAJOR spoilers within this video link.
DO NOT CLICK if you haven't played KOTOR (Knights of the Old Republic) or you DON'T want it ruined for you.
KOTOR spoiler to follow! DO NOT CLICK IF YOU HAVEN'T PLAYED AND DON'T WANNA KNOW!
RETURN OF THE JEDI: "The Shield is Up/It's a Trap!"
RETURN OF THE JEDI: "Palpatine's Throne Room" (3:09-3:30)
The Emperor even says that HE can stop the assault if Luke will only join him.
If you think about it like "the ghost of Obi Wan" went into Luke and was always in his proximity (which it was), it completely explains Luke "using the Force" to blow up the Death Star, and Obi-Wan not being able to intervene in fights Luke wasn't present for. Hell, Endor Assault is the only time Leia (or any of the main cast) gets shot by stormtroopers... twice.
The siege of Jabba's Barge however, goes off without a hitch. How in the F did a BLIND Han Solo defeat Boba Fett? /mic drop