Here's why:

* Durability: Brass is a strong and durable metal that can withstand wear and tear. It's also resistant to corrosion, which is important for an instrument that might be used outdoors.

* Workability: Brass is relatively easy to work with, allowing for intricate designs and precise markings.

* Cost: Brass was a relatively affordable material, making it accessible to a wider range of people.

However, some astrolabes were made from other materials, such as:

* Silver: More expensive and ornate astrolabes were made from silver.

* Bronze: Similar to brass, bronze is also a durable and workable metal.

* Wood: While less common, some astrolabes were made from wood, especially in regions where metal was scarce.

The materials used to make astrolabes could vary depending on the time period, geographic location, and intended use of the instrument.