Really simple. The Alexa voice assistant, also owned by Amazon, became a much more popular way to re-order products. Alexa is a multi-purpose, software based system that is infinitely configurable to order all the products that exist today and many more that may exist in the future.
The Dash, on the other hand, is a single purpose physical device. It therefore makes little sense to keep pushing the dash. It served its purpose in getting people accustomed to the idea of re-ordering from Amazon with a simple action but Alexa superseded it in usefulness.