32 bit PCI was only capable of 133 MB/s, but at the time, high speed SCSI peripherals were popular, and 64 bit SCSI cards were capable of 160 MB/s.

If I recall correctly, the PCI bus was shared and not point to point like PCIe, so if you had multiple cards it made sense to buy high end 64 bit cards for that reason.

Simply speaking - more bandwidth due to increased bus width. And later increased frequency in PCI-X slot.

But lack of useful desktop/workstation hardware and adoption of PCIe on PCs killed it.