What is the difference between EPR and rotor speed as thrust setting parameter?
While the true 'reason' is probably buried in some engineering notebooks from 60 years ago, it basically boils down to a design decision by the team designing the engine controls.  Both methods work, and both companies have a list of reasons why they think their solution is 'best'.  

P&amp;W will tell you that EPR is a better indication of engine thrust, and they're right.  But it relies on pitot probes to measure the pressures within the engine.  Flow stability means response can be slow or erratic and the probes can be clogged causing problems. OTOH, it can indicate problems (unrelated to the probes) due to engine damage.  

GE prefers to use N1.  This is a simple speed indication of the fan.  It's not a direct indication of thrust but is directly correlated.  The big advantage is that it is extremely reliable and stable.  

A good explanation is available from the Flying Engineer.