Steel guitar manufacturers basically have stopped manufacturing the Push-Pull changer mechanism (a few builders still make this mechanism). Push-pull guitars were noted for their tone. The push-pull mechanism handled relatively simple setups well.
More complicated, modern copedants and universal tunings, requiring triple-raise and/or triple-lower, showed the older, push-pull mechanisms to be inadequate and/or hard to implement and maintain.
Click on the examples below to see an animation of the push-pull changer in action.
Here's a basic parts diagram for a typical push-pull changer mechanism: