Why are road bikes (not time trial bikes) used in many triathlons? I've been watching triathlon videos lately and I couldn't help but notice that, with the exception of Ironman Competitions, only road bikes and not time trial bikes are being used. Is there any reason for that?
The OP linked 4 videos, the first 2 videos were  long-course Ironman races, where time trial (TT) bikes were used, while the latter 2 videos were of short-course draft legal races where road bikes were used. In the latter two videos the competitors were either pros or elite amateurs. These individuals typically have multiple bikes and would be using a TT bike if it was the right tool. 

Originally, all triathlons used to have a no-drafting policy, which made optimizing your personal aerodynamics critical.  Hence the rise of TT bikes in the early history of triathlons. 

Relatively recently drafting has been made legal in many short track triathlons, making TT bikes both dangerous and not suitable for the format.  When you have drafting you essentially have a road race, which a road bike is the right tool.  Quick handling is key as you need to be able to maneuver and keep your front wheel within 2-6 inches of the bike you are drafting in order to get the full draft effect.  You also need to be able to accelerate, brake, and change direction quickly to close gaps and respond to group dynamics. Groups can at times move incredibly fast, 65 kph on the flat is not uncommon for pros or elites.

In contrast, Ironman events still have no-drafting rules in place making TT bikes the right tool as its about individual riders against the wind and not responses to group dynamics.


  NOTE: the answer is intended to address the races explicitly highlighted by the OP, not all world wide triathlon events, nor the large array of amateur races where participants may be balancing a number of  considerations (e.g., cost).