I've seen that a lot of new steel and aluminum road bikes are often sold with carbon forks. On the other hand my 1990 aluminum Trek has a CrMo fork.

Is there a good reason to use different materials? Or is it just a way to advertise that part of the bike is made of a "fancier" material, and still save on costs.

the carbon supposedly absorbs some vibrations coming from the road. Thats why the seatstays and fork are sometimes made out of carbon.

Gotcha. So I guess that would explain the Al frame/CrMo fork too. Because steel is supposed to be springier than aluminum?

In bicycle forks, steel was stiffer than aluminum

Forks perform a slightly different role than a frame. Even using the same material, they were designed to handle loads differently. It got to the point where fork designers saw new possibilities with tuning the attributes of the fork design simply by switching materials. This combined with aspects of economics meant that some things designers wanted to accomplish using CF was easier and less costly than with say steel or aluminum as CF is very versatile and can be engineered from the material up.

THis is an example of the evolution of material science and manufacturing technology
through the '90s. CF has been used in frames since the '70s, usually with poor
results until monocoque frames began to be widely made in the mid '90s. In 1990
forks could only be made of steel, with cro moly blends being the high end alloy here.
A typical fork would weigh 650-800grams. Aluminum shaved 100-200gms off this
weight but it is harder to make a good Al fork and in this area of very high stress
Al was more problematic than in the rest of the frame. CF forks started showing up
in the '90s as high dollar replacements costing 5-10x what steel would cost, getting
weights down to the 500gram range. As manufacturing technology advanced, glues
got better the price of the fork drifted down. Trek switched to CF forks in their OCLV
frames in '95-'96, the previous forks being Al. The Taiwanese tackled manufacturing
and have pretty much booted the US companies out of the market, making CF forks for
<$100 so that they are cheap enough to be put on almost all road bikes costing more
than $600-800 retail. Steve

What really happened was this...

Steel bikes were prevalent in the pro peleton, until about the late 80s. Riders, especially pros, started using different materials, mostly carbon, then Titanium, then aluminum. Aluminim and Titanium forks were too flexy. Steel was too heavy. LeMond was probably one of the first to ride a titanium bike with a carbon fork. Carbon forks started making their way to steel bikes. Both lightweight and stff, carbon forks took over the fork market.

I remember Andy Hampstein was asked if he was scared his new Carbon fiber fork would break under braking stresses (you could see a CF fork twitch under braking back them), ha answered, "Yes, but I'd be afraid not to have while climbing."