If sail boats A and B are approaching each other with the wind on different sides, why is vessel A considered the give-way vessel (vessel that must take early and substantial action to avoid a collision)?

The reason is because when the wind approaches each sailboat from a different side, the sailboat that the wind approaches from the port side, which is the left side is the give-way vessel.

Whereby the wind approaches the vessel A from the port (left) side then vessel A is considered the give-way vessel

The give-way vessel is the vessel that will have to take expeditious steps to create  access for the other vessels by either changing course, stopping, or moving with reduced speed

Explanation: