Time-trial / triathlon bikes are designed purely with speed in mind, they are designed to go as fast as possible on a reasonably flat road in a straight line, the position needed on a time trial / triathlon bike is quite different to that on most other types of bicycle, in that the position is set up to be as aerodynamic as possible, keeping the back as flat as possible and the frontal profile as small as possible, but still ensuring the position is comfortable.
To achieve this position the seat angle on the frame needs to be steeper allowing the saddle to be placed further forward, moving the saddle forward allows the rider then to get lower at the handlebars, this then means that the head tube needs to be lower in order to get the bars lower. The length of the head tubes on a lot of time trial / triathlon bikes tend to be very short meaning that once the saddle is set high enough the bars end up being too low, so care should be taken to ensure the head tube is long enough.
In addition to the geometry, the frames are also designed to be as aerodynamic as possible, consequently because of the extra design involved and smaller manufacturing quantities, good quality time trial / triathlon frames tend to be quite costly.
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