Jeff is MIA for the time being so I'll answer to the best of my ability.
Izula, and RC-3 are both big sellers. That would be alot of sheaths to hand mold out of kydex, thus adding cost.
The RC-6 is a big beatch and would require alot of kydex per sheath, thus adding to the cost.
The RC-5 required a very quick sheathing solution due to the problems with Eagle. It would take a long time to spec and produce a mold for the sheaths, thus the kydex.
Some people don't like the feel of the plastic sheathing that RAT provides with some of their knives but the reality is that they're solid and functional, and that's what RAT cutlery is all about.
A healthy cottage industry has developed around providing alternative sheathing for the popular knives in the line. If you want leather or kydex there's a plethora of options available.
