Because different FireWire chipsets are not created equal. Some are better for multitrack low-latency realtime audio applications than others. Usually TI chipsets are considered to be good ones.
But it’s not that simple: my TASCAM DM-4800 (with FireWire option) does NOT like some TI chipsets, but is recommended to be used with VIA or Adaptec chipsets. You should consult your audio interface manufacturer for what they recommend.