Why is it easier to design an IIR filter in the continuous time domain than in the discrete time domain?
The term “easy” can mean simply to need less effort but can include cases where an accumulation of prior efforts can be leveraged.  

Analog filters have a long  history and someone in 1970 who was trained in analog design might consider modifying what they know to build a digital filter as “easier" than using an optimization routine written in Fortran running on their IBM mainframe.