Because VACL capturing only supports traffic in the EGRESS direction when it's being routed between vlans.
In other words if your applying a VACL between vlans 100 and any other vlan such as 200 (for example), when you apply the VACL capture feature to an L2 port assigned to vlan 100, you'll only see captured traffic FROM vlan 200 to vlan 100. Never in the opposite direction.