Why aren't contra-rotors used on the V-22 Osprey?
Instead of having a set of contra-rotating proprotors on each side, if you're after decreasing the disk loading, a span increase would also do the trick, allowing wider proprotors. However the Wikipedia paragraph starts by saying:


  Due to the requirement for folding rotors...


The main contributor to its less-than-ideal size is the folding requirement to save parking space aboard ships, since the primary operators are the US Navy and the USMC.

Adding another set of rotors or increasing the span won't help:

(Source)