Why is the left ear headphones always the one to stop working first?

90% plus of people of are right handed. Therefore most people reach to their right ear with their right hand and individually "remove" the right ear piece with their right hand. In the same motion they "pull" the left ear piece from their left ear, putting strain on the wire-earbud connection.

this repetitive pulling on the left wire eventually causes the wire to disconnect from the left bud.

to stop this left sided wear and tear: stop "pulling" your earbuds out but remove them with your hand gently.