Alan:
The simple answer is that they are attached to the aerobars. I think you mean why aren't they on the extensions with the shifters. Aerobars are usually set up this way: A bar is attached to the stem; this bar is either flat on top or a round bar like typical road bars. Instead of having drop bars curving down like a typiical road bike it curves forward and generally a bit up at the ends. The brake levers attach to the end of the bars. Your extensions with armrests are either an integrated part of the bars or an attachment. The shift levers are at the end of these extensions. Now, I suppose someone could design a brake/shift system for the extension ends but I see a couple of problems with it. First, if you have to brake it is safer to have your hands at the furthest extent to the side for stability. Why are you braking in a TT? Usually on corners or downhills where control is paramount. Second, I would guess from an aero perspective, having this kind of shifter on the extensions may be slightly "dirty" compared to the current setups. Finally, as most people in TT position have their hands wrapped around the shift levers or right behind them, it is easier to shift with your finger than taking your hands off the bar to push a lever (in road bike position, hands on hoods is easy to push the levers in).
With the advent of electronic shifting, I could see doing away with the shift levers on the extensions and moving the brake levers there (although again you want greater stability when braking) and using buttons for electronic shifting.
BT
