The primary constraint you must consider when making a FarmBot taller is the rigidity of the gantry and z-axis. If you make the FarmBot too tall without adding more v-wheels, using thicker/larger plates, and/or using larger extrusions, then the system will flex too much. This can cause problems with mounting and dismounting tools. So the reason the FarmBot is the chosen height is to ensure the system is rigid enough.
Don’t get me wrong though - FarmBot could definitely be taller. You would just need to strengthen almost all of the structural and linear guide components for it to work reliably at the larger size.