There are several reasons.
SPL meters (such as the Radio Shack meter) output a line-level voltage signal. The microphone input jack offers high gain since actual microphone signals are quite low in relation to a line level signal.
This extra gain at the mic inputs front end usually results in increased noise.
Mic inputs usually enjoy a very limited bandwidth that is designed to comply with a narrow voice frequency range that a cheap microphone would supply. The port often supplies a 5 volts bias signal also.
You require a line-in to use REW.
brucek