When triathlon first came on the scene the racers came almost exclusively from running or swimming backgrounds and had no bike racing experience. Some of the first few triathlons were disasters in terms of bike accidents, so they quickly needed to impliment something to keep people from crashing into each other. The easiest thing to do was to borrow the rules from Time Trialing and the no drafting rule in triathlon is exactly the same as it is in Time trialing on the bike. The original intention was safety.

As triathlon became a money sport and more racers were coming in that had some bike racing experience, especially at the pro level, they changed the rules as the safety issue did not seem to be there. It also makes for more spectator fun at bigger races. The first race I saw that was draft legal was an ITU World Cup race in Edmonton where I was working on the medical team. That was in 2000. I worked that event, including the world championships for 5 years straight and we did not see a single crash from the pros. So I think it works at the pro level for Oly events.

Ironman, is different animal. Triathlon had already been established and the no drafting rule implimented. So that race comes from a very you against the clock and time trialing is part of that. I don't think they'll ever change the no drafting rule at IM events and I hope they don't.