Because with a program in place to fit a Traffic Awareness System to the Puma there was nothing more that could practicably be done.
You could stop flying in pairs. The risk was not ALARP. According to HSE and Lord Cullen,
"An intention to reduce a risk ALARP, even if the required action is clearly documented and in hand, does not mean that the risk has been reduced ALARP. A risk is reduced ALARP only when the necessary control measures are in place"
Unfortunately, Duty Holders seen to thing that there is such a thing as ALARP (temporal), and there isn't.
DV