Well, I would have explained this - granted I am still early doors research wise - however, it would have been something along the lines of either
a) The limitations of the technology back then meant only one person at a time could be hacked, and their were numerous methods that could block the signal etc and various scientists within the programme were taken out or died.
b) The US lead a strike on the base that contained the device, and destroyed it without knowing - or perhaps some did - and a back up machine has been either built and / or discovered.
c) The device itself started to act on it's own, and thus was shut down and couldn't be controlled.
Long story short, once I have continued the research I will have a feasible reason for your question.