How come the double dollars option was not used during the "Money Machine" era?
I can think of two possibilities:
One, with the $150 "light", maybe somebody felt that being able to double a $150 question would make luck too much of a factor.
Two, remember that when they switched to the Money Machine, they also switched from three possible answers to two for each question; that could have played into the decision as well.
Besides, they had the "double dollars" light that replaced the "prize" light on the Money Machine during celebrity weeks (so that all of the awards given to charity were cash).