Because the technology has change dramatically over time.
A couple of decades ago:
A. Cell phones were fairly rare and air time was expensive. (short and infrequent calls). B. Used benign handsets. Most where trunk or bag units with antenna mounted on the exterior of motor vehicles. (Increased Distance from RF radiator). C. Operated in or around the 900 Mhz band.. The human body is more transparent to lower frequency RF energy. D. Volume of tissue which absorbed RF energy was much greater, thus overall exposer per in^3 was way lower.
The danger has increased because:
a. Self contained hand unit proximity to users head. (Inverse square law.. increases exposer dramatically.) b. Higher operating frequencies. (1.8 to 2.0 Ghz). (Overall RF absorption gets concentrated into a relatively small volume centered above the users ear.). c. People are using them wit greater frequency and talking for long periods.