There is a great premium to staying in the pack while cycling. And that means you have to sprint a lot. Because packs accelerate and slow down continuously. They respond to attacks and breakaway attempts, and they slow down and speed up through turns (an accordion effect). Every time that happens you have to sprint. Distance runners hardly ever sprint. They are all about aerobic and sustained effort. No redlining here lest you want to pay for it later.
Because triathlon was "invented" by runners, the cycling they do is limited to time-trialing, i.e. no drafting allowed. Many people think this is to prevent cheating, i.e. you cannot benefit from other people's work, you have to do it all yourself. Even though it makes sense at first, it is clearly not the main reason. For one, triathletes draft in swimming. For another, pack riding is not necessarily any easier than solo riding. I actually think that most people would do worse overall if drafting was legal -that is different from sneaking in an opportunistic illegal draft.
If drafting were allowed, the whole dynamics of the race would change. Those who could form packs would move much quicker, so there would be a premium for sprinting to get into a pack. Once in there you would have to work hard on a regular basis. Then attacks would develop and you would have to respond. All that means a lot of anaerobic exercise. Not good if you have to run a marathon later.
Furthermore, the sport to concentrate on when completing a triathlon is the run. Here is the event that lasts long enough and moves slow enough that one can really gain or lose a lot of time. Cycling is too fast and too nonlinear to gain much time. Even though the biking leg is longer, it is pretty hard to make up 10 minutes here.
The swim is another event where you can lose or gain a lot of time. But unlike running, you either have it or you don't. If you don't, you may as well forget it.
According to a recent NYT article, triathletes are misfits who found a something to compete in. They are pretty good at everything but not good enough at any one thing to succeed at the top. Perhaps this can be phrased otherwise: triathletes are people with excellent cardiovascular fitness but they lack the specific body type or adaptations that would make them top competitors in a particular sport.