In the early history of the orchestra there was no regular "conductor." The ensemble was directed by one of the players, except on special occasions where large forces, or several groups of players and singers at different locations in a large building, were used.
In the Baroque period the musical director was usually the keyboard continuo player, who could give hand gestures to the rest of the performers as required. That practice continued into the early classical period. For example Haydn conducted his own symphonies from the keyboard, and in one of those that he wrote for his "celebrity tour" of England in 1791-95 (No 98), he even included a little keyboard solo for himself just before the end.
When the practise of including a keyboard player in the orchestra died out, the director's role was taken over by the principal violinist, for the practical reason that the rest of the orchestra could see and coordinate with his bow movements. At critical points in the music he could play standing up, for greater visibility.
In fact this coordination with leader's bowing still happens to some degree in modern orchestras, even though the conductor is the nominally the person directing the performance.
Orchestral conductors in the modern sense began to become common in the early 19th century, but the special status of the "leader" or "concert-master" has never been abolished.
In other ensembles, the role of "leader" has moved to the analogous position of the principal player of whatever instrument usually "carries the tune". For example, in concert wind bands it is usually the leader of the clarinet section, and in brass bands it is the principal cornet or trumpet.