Don’t forget virtually all mainstream arrangers are made on a production line, and therefore to make a larger version (76 notes) would require modifying the existing line (Reducing capacity of their smaller 61 note arrangers) or building a complete new line for the larger model, which is very expensive and time consuming. (The accountants will want a 99.9% guarantee that enough sales can be made to make a profit after all the setup costs)
Roland and Korg etc already have lines for each version, so cost (The amount of sales required to make a profit) is much cheaper then having to build a line from scratch.
Hope this helps to clarify matters

Bill