Why did they build the Arks out of pig iron?

The Arks were flattened cylinders. The top two-thirds were made of pig iron. The lower third was made of martensite which is a kind of stainless steel.

Why? Why not titanium or magnalium or something even more exotic?

The first reason was seemingly trivial but actually quite important. In order to bring the Ark down safely, it had to be separated into sections. This required a space-walk. And the kind of space-walk required meant the first crew awakened would use magnetic boots. Pig iron is about the cheapest magnetic material there is.

The aft-most section was the propulsion module. Once they reached their target planet, that had to be jettisoned. The next section was the cargo section and that was to come down after the people. The front section contained the frozen people in their sarcophagi.

Why magnetic? Why not just use thrusters to separate the ship? The mission planners did not know exactly how long it was going to take to get to where they were going so to facilitate the space-walks, they knew that while propellants may evaporate, magnetism would last much longer. So they built the ship out of magnetic materials to guarantee that their magnetic boots would clamp down.

Once they landed on the planet, the mission planners wanted to give the colonists every chance to succeed so they build the Ark out of a material that could be easily melted down (the pig iron) or easily fashioned into building materials (the martensite). Titanium or magnalium or even aluminum would be too hard to work.

If things went well (and we know they rarely did), the colonists would cannibalize the entire ship to start building their new home world.