cant think of any reason except for personal preference.
My car came stock with 17". I went to 20's, then down to a few sets of 18's.
It is your call, there will be no problems with going to a smaller diameter. The only thing is the Revs Per mile of the wheel. Try and keep the circumference the same, usually around 26". If not, your speedo will be off, but that is an easy adjustment through the handheld tuner.