Because nobody had to wait in line for the first iPhone....
People had to wait in line but there was no shortage. Big difference.
I can 100% say this for sure because I was in line then. At the mall I was at there was a huge snake of line at the Apple Store and a much shorter line at the ATT store. I decided to go for the ATT store. I was like 8th in line and was lucky because that is exactly how many 8GB iPhones they received. Buy the time I bought my phone the line at Apple had completely dissipated. I was curious so I went in to see if they sold out. Nope. They had plenty of 8GB phones. Turned out Apple didn't sell out of initial stock until the tail end of the weekend end. And after then stock was never more than a day or two away -- not weeks.