Triathlon and Time Trial frames are made for speed. In time trial races the rider is not allowed to draft (ride in another riders slipstream) which therefore means the emphasis needs to go to aerodynamics and materials used. To aid the aerodynamics the frontal profile of the bike is kept narrow, helping to cut through the air, and the vertical down tube is often shaped to the rear wheel and within millimetres of the tyre.
The geometry of these is often extremely aggressive and designed with the use of aero extension handlebars in mind. Due to this, they do not make great road bike frames.