Because Beyonce told them, “To the left, to the left.” Seriously though, ever since biblical times, the right hand side has been the special side, the name Benjamin, comes from the phrase “son of the right hand side”. You may have noticed that husbands are even buried on the left side in family grave sites. There are many theories and concepts as two why but it all stems from tradition. The bride and groom always stand in the same position, the man on the left and the woman on the right (when facing the couple anyway). Because this is the position in which they are married, then they are usually photographed the same way. Also, I think it may be also that many photographers have the tendency to do what is commonly done rather than take chances to be creative. That is why wedding photography usually has the same style.

Source: Why in so many photographs of male and female couples are the men standing on the left? – Quora