In the 1920's France began the yearly tradition, and combined these three activities to work out. With a slow growth from there triathlons didn't start to get more popular till 1977. This was the year Ironman was invented by a Navy team competing for bragging rights. Since then, the races have gotten even longer and more numerous however Ironman Hawaii still holds high prestige.
Being able to conform to everyone, and being something that has held the test of time, allows triathlons to be considered a great workout.