^ good question. i think its still a pilot program with a bus from staten to manhattan:
https://www.ny1.com/nyc/all-boroughs...-new-york-city
just a guess, but i bet adding more double decker busses are a bit on the backburner.
i think the mta has its hands full right now with the new fare payment system and putting in the card readers in the subway turnstyles and so you are able to enter from any bus door.