The M-class vessels are running fairly economical. They have a large but slow revving engine and they have the waste heat recovery system, so they consume relatively less than other ships of similar size.
As to them not being sold, I think the company are very happy with the way they can service the route efficiently and cheaply (and it's hard to get a good price on old and worn down tonnage in this day and age).
As to the K and S vessel, I think they just got a very good deal on the ships they've sold. I worked on Regina, and the maintenance level on that ship was incredible! It literally seemed like a new vessel (without all the teething problems).
Regarding the B-boats, I don't think it is a big secret that they turned out to be spectacularly bad at what they were supposed to do. They were built to compete with the trans american railroad from east to west. Therefore, they are built with a huge engine (Sulzer 12RT-flex96 - same as onboard Emma M