Why are text messages limited to 160 characters?

Friedhelm Hillebrand, experimented with number of characters around 1985. He tried random sentences and questions and then counted the number of letters, numbers, punctuation marks and spaces. It seemed to always come short of 160 characters. Not sure if that would be enough he tried comparing it to other things and sure enough text found on postcards at that time was averaging on 150 characters.