Cannon - He was overconfident. Plus in the event of the shield's destruction, the DS II was to destroy the moon and escape.
Actuality - A defenseless fleet against an invincable Death Star does not a happy ending make. Same reason Tarkin really didn't blow up Yavin. Than the Rebels would have had the time the plot dictates they needed to blow up the DS.
Could we also please drop the ridiculous idea of dubbing all the Stormtroopers. Implying that the Imperial armed forces are made up of nothing but the same clones from twenty years prior is as ludicrous as when the San Jose Mercury News asked about the droids in their "10 questions we want to know about episode I". I don't remember the article fully, but they wanted Star Wars fans to write in to explain some gaps between the PT and the OT. Most were cases of watching the films with your brain shut off. The one I'm reffering to is "Why does the Empire use droids in battle in E I, but humans in E IV?". It's because the Republic IS the Empire, not the Trade Federation. Dubbing the stormtroopers adds futher confusion into making everyone think that all being hated the Empire. That all planets loved the rebellion. Its not true. I mean, what's next? Have Jango Fett's actor now play every single Imperial officer?