Paint doesn't dry on rubber...why?

Rubber contains chemicals that keep it soft and pliable. Some solvents in enamels and lacquers may attack the rubber or at least mobilize these chemicals, causing them to move to the surface and keep  the paint soft and pliable.

Try using "White-out" correction fluid or dry-brushing. 

Good question! I always wondered too.  Thanks for the explanation Triarius.

I've seen recomendations to use acrylic paints to do tire lettering. I guess it doesn't attack the tire in any way. Probably a good idea to wash the tires in mild detergant and water first to remove any surface contaminants.