I heard that Ralph Nader had something to do with the elimination of the push button because of safety; Too easy for the car to be knocked into neutral.
No, actually, if you read "Unsafe at Any Speed", you find that Nader considered the Chrysler pushbuttons safer than a lever, because they were out of reach of children who might otherwise grab a lever (being unrestrained, as most of them were at the time) and because the buttons meant no lever that could impale and seriously injure or kill passengers thrown against it in a crash (being unrestrained, as most of them were at the time).
There are all kinds of folk explanations for why Chrysler got rid of the buttons for '65. We'll likely never know for certain, but the most plausible one I've heard is that Chrysler wanted to eliminate an excuse for driver-training programs to avoid Chrysler products. At the time, most high schools offered driver training, and it was felt that the make of vehicle to which a new driver was first exposed would have a large influence on his preferences when buying.