Yarda, With the Bulli you are talking about a single part.
With the Mark-T, you are talking about two separate parts that would have to be replaced, the spike bit with the airhole, and the upper cartridge holder. If you look at the existing spike piece with the airhole on it, and try a cylinder style cartridge on it, you can see that it's too wide and a new upper part would not work on it, so a wider spike base part would have to be made.
The hole on this piece needs to be in a particular spot, which will differ for every single base unit (the bit with the screws). So, a new upper spike piece to use the larger Type B carts could only be provided without the hole, so that the user drills the hole in the correct place themselves.