It's all about revenue recognition without monthly billing the revenue for the AP is recognized fully as soon as the sale is complete. With monthly billing the revenue for the sale is recognized over 12 months and in many cases 2 fiscal years. There is a minor cost savings here as well
Once again this is about financial engineering to make WDW's numbers look better.
As to FL residents who depended on this to make their AP's work well Disney is done with you TDO thinks they don't need you anymore so Disney is doing the latest B-school thing 'Firing less profitable customers' yes it's a thing google it.
Just another reason I'm glad I'm a critic and no longer giving Disney money.
Meanwhile UNI does monthly AP payments for anyone willing to pay not just FL residents. Who wants your repeat Business hint it's not Disney