E* accounting software limitations may well be the reason for the maximum of six receivers per account. When the code was originally written in the early days of DBS, I suspect they thought six was more receivers than anyone would ever want and designed-in the limit. Times change.
Rewriting the software to accommodate more receivers would seem to be a simple task, but I would resist making a change like that if I were Charlie. Anyone who has dealt with software knows that sticking with a program that works is often a better choice than possibly introducing new bugs when "improving" it.
The number of legitimate customers who want more than six per account must be very small. I suspect this software revision is very low on Charlie's priority list.