I think I have the answer to this question.
First though, let me say that I come from a competitive running background of 15 years. Just the last 5 years I've been doing triathlons. And let me tell you, for those who have never completed nor trained for a triathlon, it is much more envigorating and more fun to train for than straight running only.
The answer is single sport runners do not do triathlon because:
1. They cannot swim worth a damn.
2. They don't have time to train for 2 other sports.
3. They are elite and/or high school or college runners dedicated to running only with much success.
4. They are wimpy and cannot handle swim workouts.
5. They are lazy and do not want to learn to swim better.
6. They have limited income and cannot afford triathlons.
7. They have just came from the couch to something atletic and chose running.
A swimmer cannot pick up running in their mid-30's and become good at it. Running well is more of a genetic gift than a learned trait, so most of you have this gift. Swimming on the other hand is mostly a learned trait, with technique and strength. That's why you can find mid-30's guys/gals pick up swimming and become quite good at it. Cycling anyone can do well with practice and a good bike.
Hard swimming workouts are actually more difficult than any
run workouts you could come up with. Sure you'll feel it more after the run workouts because of the impact your body takes, but swimming is much more strenuous to your cardio system and you recover quickly from it, unlike running workouts.
Runners already have a leg up on most triathletes with their run ability, so get off your lazy butts and learn to swim runners. Don't be such wimps. The only runners with legit excuses are runners who fall into category 2,3,6 and 7 above.
You can thank me later after you start winning triathlons.
Mequonman.