I'm not entirely sure that this subreddit is the best place for this question (not that there necessarily is such a place), because it's basically more a question of literary criticism than anything else.

I'm a former journalist who adores Orwell and is currently sitting happily underneath his entire collected works and journal, but I am by no means an Orwell scholar. I do, however, enjoy literary criticism and your question is interesting, so I'll try my best to give it a go.

First I would point you to those essays that explain why they think Orwell was prophetic. Here is the intellectual historian Louis Menand making the case in the New Yorker last year:

“1984” is obviously a Cold War book, but the Cold War ended thirty years ago. What accounts for its staying power?

Partly it’s owing to the fact that, unlike “Darkness at Noon,” Orwell’s book was not intended as a book about life under Communism. It was intended as a warning about tendencies within liberal democracies, and that is how it has been read. The postwar Sovietization of Eastern Europe produced societies right out of Orwell’s pages, but American readers responded to “1984” as a book about loyalty oaths and McCarthyism. In the nineteen-seventies, it was used to comment on Nixon and Watergate. There was a bounce in readership in 1983-84—four million copies were sold that year—because, well, it was 1984. And in 2016 it got a bump from Trump.

The entire essay is worth reading. It goes on to describe how Orwell's prophesies vary from the big, conceptual ones like the above to the smaller ones, like the use of "telescreens" at a time when Orwell may not have ever seen a television (why Menand thinks this, given Orwell's work at the BBC during the war, I am not sure. But we can be reasonably certain that there Orwell had no TV on the island of Jura when he wrote the book, which is one of the most isolated places in Europe, if not the entire planet. As an aside: I once tried to rent Orwell's house on Jura, but apparently the grandchildren of the original owners now live there full time. Visitors will apparently be greeted with shouts that they are approaching "cannibals" by the children of the house, a tradition that Orwell himself apparently inaugurated.)

While I can link to another Orwell essay worth reading, also in the New Yorker, by Clive James, sadly unless you are a subscriber all you can read is a summary.

James touches the part of Orwell that I think is very easy to forget from the vantage point of 2020. To be a socialist in the first half of the 20th century, and Orwell was decidedly a socialist, meant to exist in a realm of left-wing thinking in which the Soviet Union, including the Soviet Union under Stalin, was viewed as an ally, a model, and even inexorable future that society was destined for under the Marxian dialectic as sure as an apple will inexorably fall because of gravity.

As James writes:

So here, already formed, were two of his three main political beliefs--about the awfulness of capitalism and the need for an egalitarian alternative. There was nothing uncommon about them except their intensity. The third belief was the spanner in the works. Again, he was not the only one to have figured out that the Soviet Union's vaunted Socialist utopia was a put-up job, but nobody ever expressed his revulsion better or more lastingly than Orwell, who got it right without ever having to go there.

He went somewhere else instead. Discovering in Spain, from the behavior of the Russian representatives and their Communist adherents, that the Soviet Union was as implacable an enemy of his egalitarian aspirations as Nazi Germany or Fascist Italy, he developed the idea that it wasn't enough to be against Mussolini and Hitler: you had to be against Stalin as well, because the enemy was totalitarianism itself. [...] If he had stayed in Spain any longer he would almost certainly have been murdered. The anarchist group in whose ranks he had fought, the POUM, was being liquidated on Soviet orders, and his name was on the list. [...]

[...] his three main beliefs--capitalism was a disease, socialism was the cure, and Communism would kill the patient [....]

Both Menand and James make a point of noting that part of what Orwell was "predicting" was the fact that the Soviet show trials were a product of extensive torture. (In 1984, Winston Smith is not argued into believing that 2+2=5, he is physically tortured by rats.) While your question reflects our current understanding of this, that these were authoritarian regimes, this was widely denied at the time and the socialist left largely insisted that the confessions were genuine and that those killed in Stalin's purges were somehow actually guilty. The historian Robert Conquest, who wrote a then-much-derided book as late as 1968 estimating the high toll of Stalin's purges was encouraged by Kingsley Amis, once Conquest's estimates of the death toll had been vindicated decades later, to subtitle new editions of The Great Terror as "I Told You So, You Fucking Fools".

Orwell, sadly, did not live to see such vindication.

On the contrary, Animal Farm (1945), a critique of Stalinism, essentially cost Orwell his relationship with his publisher (and much of the British left) and faced incredible difficulty in getting published at all.

It sounds bizarre to say so, but part of Orwell's popularity was directly encouraged by the CIA, which after his death recognized the power of Orwell's anti-communist arguments.

The other element that I think is impossible to escape, although this is where I go more into my own opinion (although many, many others have certainly written about this) is that Orwell I think more than anyone before him was able to describe the relationship between language, politics, art, and propaganda. He coined, for example, the phrase "all art is propaganda". His job at the BBC during the war was literally to be a propagandist. While others, going back to what I have heard described as as Napoleon's invention of modern state propaganda, understood the power of words to influence politics, Orwell observed and described the influence of that better than anyone before him in a way that while not perhaps "prophetic" is objectively useful. Language was used in "Orwellian" terms before Orwell, but that adjective applies because he was the first to describe the malady (much the same way diseases identified by a doctor are often named after them).

In my own opinion, I wouldn't call Orwell a prophet. He was, at heart, a journalist, and his great talent was describing truth as he saw it with no regard for the social pressures on him to do otherwise. His style is almost absurdly plain spoken. I think he is, to borrow a phrase, great because he is good. As a good man living in a profoundly evil period of history he was able to write good things that I think have and that I hope will stand the test of time and that remain incredibly useful to anyone who wants to understand modernity, politics, and the human condition.

edit: aside from his own books and essays, which everyone should read, for a short introduction I would recommend the BBC's In Our Time episode on Animal Farm: https://www.bbc.co.uk/sounds/play/b07wgkz4