An in-depth look at the design of the MacBook suggests that Apple simply might not have had room for a FireWire port. Apple takes its hardware design very seriously, and it’s very likely that in attempting to achieve certain design criteria, compromises had to be made.
“The unibody construction, along with the decision to make the hard drive and battery easily accessible essentially determined the rest of the internal layout, and you wind up with motherboard that doesn’t span the length of the notebook. So, all of the ports have to go on one side, and the limited space means you get three.”
You can check out the full analysis over here.