Elite athletes may well race Duathlons prior to the triathlon season starting…the water is too cold for most people until April (ish) and so triathlon races do not normally start until then. A similar argument applies after September when it starts to get colder again. So there are commercial pressures around this but also the demand for pre-season racing to test out early season form.
The same argument will no doubt apply to many Age Group athletes.
But this explanation does not cover everyone who competes in duathlons.
Some might see them as a way of doing quite well with a little bit of bike training. If you are a very good runner it’s not so hard to become an excellent duathlete..much harder to become an excellent runner and much much harder to become na excellent triathlete. I think this is a reason for quite a lot of good runners going into duathlon…there seems to be many more former runners than former cyclists doing well in the duathlons.
For others it might simply add a bit of variety or novelty or represent another kind of challenge. Perhaps a challenge as a stepping stone to a triathlon. For example I strongly suspect that many of the ultra distance competitors in the London Duathlon this September are doing it as part of IronMan training. So they are also part of triathlon training.
Maybe the variety angle is important if simply you can’t swim!
Perhaps also it’s a good way to avoid injury by cross training or to help putting too much running stress on a body if recovering from injury.