While the "just too dangerous" argument is the standard excuse, my guess is that like many of the less mainstream events (after all, how may people were really riding tandems, never mind racing the darn things by the 70s) it was eliminated because it was just too obscure to warrant the time and specialized equipment needed to field the event. The 100 km Team Time Trial was also eliminated from the Olympics in 1992 for what was most likely similar reasons. Conversely, they've since added mountain biking, BMX and some other track events.
It's also likely the UCI has had a hand in pushing for these changes, noting they too try to structure their rules and sanctioned events to mirror contemporary spectator and enthusiast trends.
For example, the men's and women's individual pursuits & points races along with the men's Madison will be gone from the Olympics in 2012. These track events are being replaced by men's and women's sprints, keirins, team sprints, team pursuits and omniums. So, while there were seven events for men and three for women at the Beijing Olympics in 2008, there will be five events each for men and women at the 2012 games in London. It's a sign of the times.
As for the Paralympics, as you note it somewhat blows a hole in the "too dangerous" argument as there is absolutely no difference -- from a safety standpoint -- in the risks and speeds involved. However, it's clearly a situation where tandem track events are a perfect fit and very popular cycling event in the Paralympic movement. While I don't have a clue how you'd even find crash statistics, I'd guess there are proportionately just as many in the Paralympics as there were when the UCI and IOC sanctioned the events at the World Championship and Olympic levels.
Last edited by TandemGeek; 03-16-10 at 08:29 PM.