MIDI interface uses opto-isolators to prevent ground loops or voltage spikes. Moreover, MIDI was conceived ~10 years before USB, and its bandwidth is far more limited. Thus USB/MIDI "cable" is not just a cable, but a specialized device, which should contain a buffer, in case of the data coming from USB side faster, than MIDI side could consume it. If this buffer is too small, the data can be malformed, causing errors or being discarded.