"Why would Apogee use USB rather than Firewire for its recent Mac audio products like Symphony I/O, Duet 2 and ONE?"
Historically, Firewire has been the superior interface audio format on the Mac. However, since the release of OS X 10.6.4, USB Audio is more stable, offers lower latency and provides the ability to connect your audio interface separate from an external audio hard drive on your Mac.
Apple Logic configured at a 96kHz sampling rate and 64 buffer setting, Duet USB's analog in to analog out latency is 4.2 milliseconds, while Duet Firewire's latency is 5.3 milliseconds.
Most audio software developers agree that recording audio to an external Firewire hard drive renders the best results. When the audio interface is connected to a different computer port than the audio drive, system performance is more stable. Thus, it's preferable to use a Firewire hard drive and a USB audio interface.