I saw that too and did a quick search. I came across about it. Two items stood out.

First, the opening:

What to Know The noun usage of sudden has all but been abandoned by history and the word has become bound to "all" in the idiom "all of a sudden," meaning "suddenly, or sooner than expected." There's no grammatical reason why the correct phrase is "all of a sudden" vs "all of the sudden," it's just the recognized form of the idiom English speakers have accepted.

Then this:

Here's where it gets weird: there's no clear-cut grammatical explanation as to why we use the article a in the expression instead of the. In the past, both articles commonly preceded the noun sudden (meaning "an unexpected occurrence, need, or danger") in phrases formed with of having the adverbial meaning "suddenly."

Then I got busy and sidetracked and forgot about it until i saw your post.

suddenly, all of a sudden, or on a sudden

Welcome to The Mandela Effect.

"all of a sudden" is an idiom so is still correct while "all of the sudden" is a bastardization of that idiom that only a retard would use.

idioms are exempt

With "Sudden," you have a very precise moment in time. With this idiom, you have all of that moment, or All of a sudden.

In an instant, he was gone. In all of an instant, he was gone. All of an instant, he was gone.

All of a sudden, he was gone.

Just my take on it.