one of reason may be the threshold value

The minimum value is 4 mA is because to avoid any confusion
of out put at lowest value.

the lowest value is not 0 to create a difference in between
broken ckt or any other error in ckt and actual min current
value. 
b'coz for broken ckt or any other fault in ckt the current
o/p will be 0. means no current. but when it is 4 , then it
is the min o/p of the ckt and the ckt is running properly
both is confirmed. 
the upper limit is a conventional one, no such reason behind
it. but it is a value  practically can be achieved by the
designer easily. 
4-20 ma could be taken it is followed by all manufacturers.

4 milliamp is used as span lower limit, because some 
 elements on the loop require power from the loop and
 therefore a minimum level of power must be available

1. to obtain live zero
2. noise reduction
3. no current drop
4. longb tranmission

4-20mA :-
          * Easy to find the open circuit(because this 
signal level starts from 4mA, if the circuit in open the 
output shows 0mA, so we can find easily the open circuit).

          * Easy 0 to 100% segregation
4mA  - 0%
8mA  - 25%   12mA - 50%  
16mA - 75%   20mA - 100% 

          * All manufacturer following the same standard.

0-20mA :-
         * If it starts form 0mA, not able to find open 
circuit easily.

3-15mA :-
         * Less linearity, because 3mA is 0%, 15mA is 100% 
so the deviation b/w 3-15mA is lesser than 4-20mA, So that 
it reduces the accuracy of the measurement. 



So that 4-20mA is preferred over 0-10v