Check the usual suspects: Termination, Where is it on the SCSI Chain, What SCSI ID it has, Formatting, System Install instead of dropping in the System Folder to copy it, Blessed System Folder, and so on.
Not all ZIP Disks will boot a system, and not all systems will accept a Zip Disk Drive as a boot drive. A Jazz Drive works a lot better in this regard. The Zip Drive Drivers is not that necessary as they are one of the later set of driver extensions to load on most systems.
The problems here are you not mentioning what kind of Mac System you are using the Zip Drive on, and what System/OS the Zip Disk contains. In the very least, the Zip Drive should be the first device on the external SCSI Chain, with Termination on and a low SCSI ID number (the Internal Hard Drive is usually 0, so the Zip Drive should be 1.) If possible, you may need to use Drive Set Up to format the Zip Disk if its allowable, not all Drive Set Up programs support Zip Drives. Just using "Erase Disk" or "Format Disk" from the Desktop Menu will not add Boot Sectors to your Zip disk. I believe that the Iomega Utilities from the Zip Drive Floppy Tools Disk can format a Zip Disk and give it Boot Sectors, use it if you have it.