Because the ADA as a national governing body for dental students is embarrassingly out-of-touch with the struggles and ramifications of what it means to wait 5 months for an acceptance after getting early interviews. There is absolutely zero reason to restrict dental schools from notifying applicants for acceptance within a reasonable time frame - and it can probably be linked more directly to the ADA wanting more and more money out of you on their application portal, since students naturally have to apply to more schools with more uncertainty on this restriction. If you look at every other health professional occupation, they are flexible with their student applicants, giving them abilities to make an Early Decision commitment on a school they really feel strongly about, and notifying them on decisions within a month or so of their interviews. The ADA is so behind medicine, pharmacy, optometry, and law in terms of not abusing their own.
The reality is it always goes back to money. Dental school tuition, its standardized exam fees and license board fees, and application fees have exponentially increased even against the cost of inflation since 2000, and the ADA does everything in their power to suck every dollar from your wallet even without an acceptance. I hope the penguins at the top of this organization do some some serious soul-searching to fairly represent ASDA in this process, in a way that doesn't suck thousands more dollars than what students can budget for.
Then it goes back to time, where since you don't know until December, you naturally have to assume you're rejected, therefore enrolling in more classes (and shock! more money!) in case you get waitlisted or rejected.
It's a ridiculous process and the ADA deserves massive criticism for the way they exploit their future dental professionals. Of course, it shouldn't surprise you that old dentists as board members of the ADA are out-of-touch: dentists from 30 years ago had far fewer debts to pay back, fewer challenges with a revolving healthcare insurance system for their patients, and easier times starting up as a private dentist taking loans from banks.