Why does a game object fall slowly when rigidbody2d.velocity.y is equal to 0?

Question

I'm new to Unity.

rb2.velocity = new Vector2(speed * moveDirection, Y); 

When I set the Y in new Vector() to 0, the character falls slowly because of (I think) the gravity. But when I set the Y to rb2.velocity.y, the character falls ordinarily.

Can you tell me the reason? Thx.

public class SecondCharacterController : MonoBehaviour
{
    public float speed = 1.0f;
    private float moveDirection;

    private Rigidbody2D rb2;
    private Animator anim;

    private void Awake()
    {
        anim = GetComponent<Animator>();  
    }

    private void Start()
    {
        rb2 = GetComponent<Rigidbody2D>();
    }

    private void FixedUpdate()
    {
        rb2.velocity = new Vector2(speed * moveDirection, rb2.velocity.y);      // THIS LINE
    }

    private void Update()
    {
        if(Input.GetAxis("Horizontal") != 0)  // Moving
        {
            if(Input.GetKey(KeyCode.A))
            {
                moveDirection = -1.0f;
                anim.SetFloat("speed", speed);
            }
            else if(Input.GetKey(KeyCode.D))
            {
                moveDirection = 1.0f;
                anim.SetFloat("speed", speed);
            }
        }
        else   // Idle
        {
            moveDirection = 0.0f;
            anim.SetFloat("speed", 0.0f);
        }
    }
}