Because it is negative.and we know that negative charge move to positive.

Electron Flow Direction vs Current Flow Direction
Understanding the direction of electron flow in a circuit can be confusing for many people, as it is opposite to the conventional direction of current flow. Here's why:

Conventional Current Flow:
- In conventional current flow, we consider the movement of positive charges from the positive terminal of a voltage source to the negative terminal. This is the direction in which current is said to flow in a circuit.
- This convention was established before the discovery of the electron, when it was believed that positive charges were the carriers of electric current.

Electron Flow:
- Electrons are actually negatively charged particles that move from the negative terminal of a voltage source to the positive terminal.
- Due to this movement of electrons, current flows in the opposite direction to the movement of electrons in a circuit.
- This is why we say that electron flow is in the opposite direction to current flow.

Why the Conventional Current Flow is Used:
- Despite the fact that electrons are the actual charge carriers, the conventional current flow notation is still used in electrical engineering and circuit analysis.
- This convention simplifies calculations and circuit diagrams, making it easier to understand and work with circuits.
- Therefore, it is important to keep in mind the direction of electron flow when analyzing circuits, even though current flow is considered in the opposite direction.