Blame Steve jobs then. We just bought these MacBooks mid-last year.
It's possible, but not likely since Apple is one of the champions of Firewire. It's also likely you had a higher-performance or higher-capacity drive in the USB enclosure. I found a comparison (from a company that promotes USB) that used the same IDE drive in two enclosures:
Read Test:
* 5000 files (300 MB total) FireWire was 33% faster than USB 2.0
* 160 files (650MB total) FireWire was 70% faster than USB 2.0
Write Test:
* 5000 files (300 MB total) FireWire was 16% faster than USB 2.0
* 160 files (650MB total) FireWire was 48% faster than USB 2.0
That is aside from the various other benefits of Firewire, which may or may not apply to you.