Probably because he might have access to a canopy from another make/model at a much lower cost than the MN canopy.
Just my personal position, but I really hate threads like this where the the OP doesn't give all the necessary information. That said, no two tubs from different makes or models are ever the same, so making it fit will require either a master fibreglasser or body builder. In either case the result is going to be a compromise, or ugly, or both.
Maybe go the whole hog: