In order to understand why an ancient smith used particular metals, we'd need to be able to go back in time and read their minds. All we can do is look at the end product.
We do know that until fairly recently, knowledge of steel's actual makeup was little known. We're pretty sure people in the past knew, for instance, that carbon was an essential element of steel, and that too much is a bad thing. But did they know that for different purposes a .95% carbon steel was better than a 1.05% carbon steel? I have to be skeptical about that.
Many of the older accounts of blacksmithing discuss selection of materials based on intended purpose. If a smith was working with unknown scrap and needed to make a spring, a piece of the appropriate size might be selected and tested to see if it would harden--if so, it might be used.
Welding pieces of scrap into a larger piece for use when large piece of stock was needed was a common practice. Using the cementation process to produce steel is easier if working with smaller pieces, to later be welded into larger pieces and worked repeatedly to produce a piece of relatively homogeneous steel. The problem with this is that every batch was different. If a smith took a number of pieces of scrap and worked up the stock for a larger product, the end result was something like what we know as pattern-welded damascus. (I actually had this result a couple of times back when I was a kid.) Homogeneous steel wasn't available until the introduction of the crucible process in the mid-1700s.
An analogous process sometimes occurs when wrought iron (for example, in the Viking and Celtic blades) was used with high-carbon steel. the materials seem to have been selected for their qualities--steel for hardenability and springiness, and wrought iron for toughness. Carbon migrates from the higher-carbon material to the lower-carbon material.
Both of these processes make it impossible to determine if different steels were used in order to create the patterns.
Then there is the problem that very few people who have ancient blades are willing to submit them to testing in order to analyze the actual carbon content of the steel. We can only work by analogy here, by assessing the results of modern experiments. In one example of piled stock I did, there was enough carbon migration between the high-carbon steel and the wrought iron that I ended up with a piece with a carbon content of about .6%, which we'd barely call high-carbon steel. But the barstock showed a pattern when etched.
I suspect similar things occurred 2,000 years ago but all we can do is analyze the surviving blades--we'll never know the reason for the smith's selection of raw materials.
Hopefully, that will be enough babbling to get someone who really knows what they are doing (instead of just a dilettante like me!) to come out of the woodwork.
