Because they are more durable than ordinary matel. Titanium and nikel have the unique properties, shape memory and superelasticity. That is to say, They can restore to original shape after being bended without undergoing deformation. And they have enormous elasticity, some 10-30 times that of ordinary metal.
They are with good quality. They are better than other ordinary metal because they are corrosion resistance. Maybe that's the reason why so many people like them though they are much expensive than glasses made by other kind of materials and they are of less style.
titanium glasses and nikel glasses are very good because they are of good quality. They can last longer when compared with other kind of glasses. Many people don't like rust on the frame. But the glasses made by ordinary metal often have stains of rust which is difficult to get rid of. But titanium glasses or nikel glasses won't cause this problem. But I seldom choose a pair of titanium glasses because they are too expensive for me and they are not stylish. I am not allergic to metal glasses or plastic glasses so I can choose any one of them.