First, I was doing triathlons before draft legal triathlons existed or at least were common.
Remember draft legal races are pretty much limited to ITU professional races.
Here are a couple of reasons as to why drafting is not popular in triathlon IMHO:
1) The sport developed with no drafting so it is in the DNA of the sport. ITU instituted draft legal in an attempt to make it more popular. Not sure if it did that or not. The most popular event is the Ironman--drafting is not allowed.
2) Many see the sport as an INDIVIDUAL one. Drafting obviously reduces that.
As for swimming, I think for one thing the advantage in swimming is way smaller than in cycling. Yes water is more dense, but resistance increases exponentially with velocity and swimming velocity is just not that great. The link below gives time savings over 400m.
http://europepmc.org/abstract/med/9710869
This paper shows a 3-5% performance improvement.
https://www.researchgate.net/profile/Jean-Claude_Chatard/publication/10678070_Drafting_Distance_in_Swimming/links/561e805b08ae50795aff0442.pdf
3% is far less of an advantage than the upwards of 40- 60% of power savings you can see in cycling drafting (depending on position. 3rd person in a paceline saves more energy than 2nd person. 4th person and back in a paceline does not get much more of an advantage).
https://books.google.com/books?hl=en&lr=&id=msdT4iQ50cgC&oi=fnd&pg=PA147&dq=drafting+in++cycling&ots=Uejj1TTe9f&sig=j09d2dxln5_yBrj5oVP_a-xYfLk#v=onepage&q=drafting%20in%20%20cycling&f=false
Back to drafting being okay on the swim, well it would be very challenging to enforce such a rule in a race. You would need officials in some sort of boat on the swim course and it can be crowded already.
I also think there is a safety issue in drafting or riding in packs in cycling. Few triathletes come to the sport with a bike racing background. Going 25 mph and one squirrelly rider can be bad news for everyone else. Even pro triathletes have issues with bike handling in a pack. The group rides in Boulder from what I hear are always a little cautious when the triathletes show up.
Running drafting is not big either. At marathon pace it is about 2%. (A nice piece on Pugh's work).
http://www.runnersworld.com/sweat-science/does-drafting-help-in-running