Why do Manufacturer's decide to make 4 or more versions of the same piece of machinery.

I was looking at a tractor catalog  (on line)  looking for 1/2 dozen or so different unrelated parts. Manufacurer had 4 different versions of same part.

Sorry,, Manufacturer's built s_ _ _tpot of different versions of same tractor, then dealer's complain about all the parts needed ??  I fail to see the advantage of making 4 different versions of a door handle assembly.  A door latch is a door latch. It holds the door closed when operating the D _ _ _ tractor, and opens the door when I want to get out!

here's a wild thought,,, how about 1 door handle that fits every tractor in any 1 given series ???  

Yah,, I know,,, not nearly complicated enough !!