We now know why the Apple and AT&T preorder system for the iPhone was paralyzed on Tuesday. Apple says the one-day preorder volume was the highest it's ever taken. Together with its carrier partners, Apple took preorders for 600,000 iPhone 4 handsets on Tuesday. The sheer number of orders overwhelmed the online ordering system causing it to crash repeatedly throughout the day. READ MORE: Cnet.comApple: 600,000 Iphone 4 Pre-Orders on First Day!