Here's why:

* Silicates are the most abundant minerals in the Earth's crust. Feldspar and quartz are both common silicate minerals. They make up a large percentage of igneous, sedimentary, and metamorphic rocks.

* Copper and iron are metals. While they are important elements, they are found in lower concentrations in the crust compared to silicates. They are often found in concentrated deposits, but these are less common than the widespread occurrence of feldspar and quartz.

Think of it this way:

* You're likely to find sand (made of quartz) or granite (containing feldspar) on almost any beach or mountain.

* You're less likely to find a big chunk of copper or iron just lying around. You need to specifically mine for these metals.

In summary: Silicates like feldspar and quartz are the building blocks of the Earth's crust, while metals like copper and iron are found in more concentrated deposits.