Why is a current signal preferred to a voltage signal for long analog transmission?
Actually what matters for immunity against noise is the power that is needed 
to disturb the singal.

I.e. a current signal at an input with nearly zero impedance is just as bad as
a voltage signal at an input with nearly infinite impedance.

What is needed is a receiver with non-zero as well as non-infinite impedance so that the signal involves some power.
I.e. 


if the information is coded as voltage, there still should be some current flowing into the receiver and  
if the information is coded as current, there still should be some voltage across the receiver.  


So both cases are similar, but you just have decide whether is is better to code the signal
as voltage or as current (another alternative would be coded as power).
For measurement purposes voltage or current signals are most appropriate.

A good wire for a current signal just needs to ensure that no current is lost (or inserted), 
i.e. ideally no leakage, i.e. perfect isolation. This can be accomplished in practice quite well.

A good wire for a voltage signal needs to ensure that no voltage is lost, 
i.e. ideally no voltage drop, perfect conductance along the wire. 
Unless you are using a superconductor this is almost impossible to accomplish in practice.







simulate this circuit &ndash; Schematic created using CircuitLab

In any case receiver resistance should be well above 0 and well below infinity.
It's easy to have the isolation resistance practically infinite.
It's practically impossible to have the series resistance 0.

Therefore if the signal has to be sent down some distance along a wire it is better to 
use a current signal than a voltage signal.