Why is matter considered a cause?
Because without matter nothing comes into being or is destroyed in the visible universe. A rock, a tree, an animal or a human must include matter in its makeup. A cause is define as something upon which others depend for their being or becoming, therefore, matter is a cause.
The matter we experience has form even atoms and energy. It is always the matter of something or someone, the stuff out of which everything is made. Einstein equation(E=mc2) means that matter and energy are two aspects of the same reality, so even energy has some form.
Aristotle reasoned that when something changes, (e.g. water into air, or we would say water into water vapor), there must be something underlying reality which persists which he called 'Prime Matter". This matter has neither substance or quantity, it is invisible and eternal, it is pure potentiality. This prime matter which he envisioned can only be such in the mind of God.
Today science is getting closer to the concept of prime matter. Astronomers have observed that our universe is constantly expanding yet remains constant. How is this possible? Science estimates that 68% of the universe is dark energy and that 27% is dark matter, so we only see and understand only 5% of the universe. The Quantum theory says that empty space is actually full of temporary (virtual) particles that continually form and dissappear. I think that we can make the assumption that "dark energy/matter is constantly being created to maintain the universe in a balanced state. In other words, it is God who constantly maintains the universe in existance because it is dependent on Him who is its prime cause.