why would I lose 3 clutches in 2 years on 1993 ranger splash 4x4 without overloading it?

Was the friction material on the clutch disc wearing out, throw out bearing going bad, do you have a young person inexperienced with manual transmission and clutch operation or is something else going on? Please be more specific.

First clutch replacement included, pilot bearing,clutch, pressure plate, and slave cylinder, but lasted about 4months and began sliipping again...

clutch disc did not show any wear nor was the throw out bearing wearing. Only drivers were adults with many years of experience driving straight stick transmissions. The vehicle achieved excellent gas mileage at near 30 mpg on highway since I had installed manual locking hubs a few years back, but the slippage just began all of the sudden. No sign of oil or debris in clutch area when being replaced either...

Makes me wonder if there is an issue with the master or slave cylinder no letting the clutch fully engage. Another possibility although remote could be a wrong part that was similar to original equipment causing the problem. Were the parts used for the repair genuine Ford parts or aftermarket?