I don't know if this is absolutely true but I heard it from a very reliable source some years ago who was connected to the Fleet Service Division.
One of the main reasons Mack stopped manufacturing fire engines had to do with the contract they had with New York City. The apparatus warranties placed a huge demand on Mack for repairs. The FDNY policy that ran new rigs into the ground at the busy companies and then transfered them out do the hinterlands almost guaranteed that rigs, new or old, we're constantly in the shops, with Mack footing the bill. It was a big money loser and Mack just didn't need the aggravation so they discontinued manufacturing fire engines. Don't expect to see them back anytime soon.
Again, I'm not so sure about this story, and could remember it wrong, so any comments will be taken with value.