Because when input is on 4mA output voltage is 1V. So you get output 1-5V for input 4-20mA. Some pic have differential input, so you still can get all 8,10 or 12 bit. Or you can in software subtract value to get 0 reading when input is 4mA, but then resolution is lower.
Also you can use OAmp with offset to get 0 on output when input is 4mA...