Why does carbon dioxide not form addition compounds?
A fairly simplistic argument is that in CO, the lone pair on oxygen is of a much lower energy than the lone pair on carbon (because oxygen has a greater effective nuclear charge and therefore pulls electrons closer to itself). Therefore it is quite rare (very rare indeed) to find any compound that has the oxygen acting as a donor atom; CO only really reacts via its lone pair on C.

Carbon dioxide only has low-energy lone pairs on O, and is consequently a lousy Lewis base.

One could easily refine this by bringing in MO schemes, but at 3 a.m. in the morning, this is good enough to convince me.