This is complete speculation on my part, but I wonder if part of the reason why they stopped offering deferrals is because whatever number constitutes "too many" people signed up, struggled with or did not train, and then decided "too close" to race day that they had no chance of being ready so they deferred. Thus possibly costing runDisney a registration in the current race and in the deferred race.
I mean except for the deferral part, much of that describes my first training season for a Disney race. Started training on time, got lazy, skipped too many runs because of laziness, unwisely tried to make up for lost time, and so forth. I still finished, but when I look back at my training as a very inexperienced runner, I really see just how much I've learned since then.