Aerodynamics and a more aggressive riding position. Steel framed bikes with round tubes aren't as "aero" as modern tri-bike.

Cervelo is a popular tri-bike manufacturer. They wind-tunnel test their stuff and they have a lot of interesting information on their site that confirms some of the pro-vintage statements you see here.

For example, weight, even on a hilly course isn't nearly as an important a factor as people make it out to be. Aerodynamics is everything and body position is the most important factor in how aero you'll be. Wheels can make a big difference but modern wheels can be put on an old bike, though they might look strange. An aero frame has less impact than aero wheels but can save you a minute on a 40k ride.

And virtually nobody uses a traditional speedo in triathlons anymore ;-)