IDE to USB can't see the disk

There is no real reason why an IDE disk would not work, but there are some reasons why there might be quirks.

There were some oddities in terms of sizes and LBA modes, but nothing that would break anything or prevent it from working.

The main thing though is that IDE disks did have a Primary and Secondary jumper, sometimes also called "master" and "slave", that dictated which connector on the cable you fitted them to and which one showed up to the system as the primary disk. It is possible that if they were the secondary disk in a system and you had a USB caddy that does not support the cable and jumper selected primary or secondary mode then it may simply not "see" the drive. Essentially the drive is expecting to have a particular pin pulled high or low to tell it that it is the one being addressed. If the controller does not use that line then the drive will stay silent and not respond.

Alternatively Windows may not have assigned a drive letter to the disk, or the disk may have somehow been corrupt. Using Disk Management would have told you whether Windows itself could see the disk which would have told you whether the problem is the disk format or the USB IDE enclosure.