Steel is an extremely tough metal but the cost is weight. You can build a bike to be bullet proof, capable of a million miles for next to nothing but it will be extremely heavy.

In engineering, strength in your number one concern with trying to use as little weight as possible. Where extra strength is needed, weight can be sacrificed. However, with clever engineering principles, weight can be added without effecting the performance much.

Take my wheels that I am running right now, the Dura Ace WH7700. Nimples need to be used on wheels and are large masses in the spoke equation. However, Shimano decided to mount them on the hub instead of rim. This reduces the rotational momentum by bringing mass closer to the center. With the cassette, I am sure they have similar reasons. Larger gears have a larger axis resulting in a higher momentum, so the mass of the gear is reduced by applying titanium, however, the smaller gears have smaller axis so the rotational momentum is less effected by a little mass addition.

The material the part is made of only tells a third of the story. You can give me a peice of wire, and within 10 minutes, I can change the materials properties without adding any extra elements and make something that is strong and ductile, or make it weak, stiff, and brittle.

However, the density of a metal is not always the best figure of overall weight that the part will finally weigh. Take the age old question steel vs aluminum. Aluminum is approximately 3 times lighter then steel, however, steel is 3 times stronger. This means that your going to have to either have thicker tube walls on the aluminum, or just overall bigger tubes to have a frame with the same strength of a steel one. Over all you end up with the same weight (although this doesn't really apply to the cassette)