This is an entertainment event. The organizers are probably interested in making some money or at least not showing a loss. They also support a charity as well with part of the proceeds. I am sure they want to encourage participation and keep ridership up in order to keep the event sustainable.

The rules regarding the bikes do not exclude any brand. The OP's bike is very appropriate and fits all of the guidelines laid out in the regs...without changing a thing except perhaps the seat. They have a bike show for the purists but again, the event itself appears to be not exclusive to any specific brands.