The simple reason is that the Firewire architecture requires more physical space than a USB architecture (having something to do with the fact that USB devices are either master or slave but FireWire is always able to be both). In order for the new iPods to be so thin, they couldn't have a fully-functioning FireWire port. This is also why you can charge the 5G iPods over FireWire, but can't sync--they have the ability to get power over the cable, but not anything more sophisticated.
It's likely also cheaper, but not significantly so.