Why is RS422a more immune to electrical noise than RS232?
note: answer is about current loops, not RS422A  

Current loops have a number of advantages over voltage lines. A 4/20mA loop, which uses 4mA and 20mA to represent two logical levels, allows for instance to detect cable breaks, since in case of an interrupted cable the current will be zero. Another advantage is its noise immunity, which is important in industrial environments with high power machines. The reason for the noise immunity is the low impedance of the line; a 250\$\Omega\$ resistor is enough to create a 5V level on the receiving side.
Voltage lines on the other hand are high impedance, which results in low-power. (Here the current loop has the disadvantage.) High impedance makes the line noise sensitive: since the noise energy has nowhere to go (due to the high impedance) it can easily build up a relatively high voltage. The low impedance of the current loop ensures that any injected noise is immediately drained to the load, so it can't build up high voltages which would disturb the loop's current.  

edit
The high power requirements are the main reason current loops are only used in industrial environments. Low-power voltage lines typically use differential signalling to increase noise immunity. Explained in this answer.  

edit 2 (re Russell's comment)
RS422 is differential (thanks Russell), multi-drop, and improves on RS232's speed. I wasn't sure about the differential thing, but it's normally not used in current loop systems. Anyway, RS422 is voltage driven.