In my expereince the very best Triathletes are swimmers first and then either cyclists or runners. Anyone fit/strong can ride a bike reasonably fast and run for an hour. Swimming fast enough to be competitive is a fine art that is not easily learnt.
What he said.
Swiming might be the shortest discipline, but with bad technique it's the one that kills you. The best swimmers will glide effortlessly through the water using mainly their arms, saving the legs for the bike and run. Mere mortals will splash about for a while and drag themselves into transition gasping.
I'm not saying that cycling and running are without technique, but there are a lot more people who are naturally reasonably efficient cyclists / runners than swimmers.
Are triathletes cyclists? I was a cyclist who wanted to meet girls and the swimming and running clubs were full of them!