Explain why the conversion ratios used between the si and imperial systems are not nice, round numbers like those used to convert within either system

Because within each system, units are defined in terms of others in the same system using nice round numbers, but no unit in either system is defined in terms of a unit in the other system. The one exception is the unit of time, and there, the relationship is a nice round number: 1 metric second = 1 imperial second.