Why is it easier to design an IIR filter in the continuous time domain than in the discrete time domain? Given that the goal is to approximate the ideal filter with an IIR filter, why would it be easier to design it in the continuous time domain than the discrete? 

This paper says:


  Discrete
  time IIR filter design can be a very complex procedure in discrete time domain. Therefore, transformations have been
  developed to use well known design methods in continuous time domain. 


This may be a very trivial question (sorry if it is), but I don't see why it is easier.

Perhaps I could be asking this in a better way: 
Why do we need to translate filter specifications from discrete to continuous time and then do the calculations in continuous time? Why not directly in discrete time?
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.