Howdy! This has been discussed at length a number of times already, which you may not have known. There's a lot of people on both sides of the fence, so if you start searching for threads on here you're very likely to run into a bunch of these discussions.
It really comes down to just a few things: it's colorless; it turns virtually every instant and sorcery into a powerhouse when left unchecked; most effects repeated for free become quickly backbreaking; and its an artifact.
If Panoptic Mirror weren't colorless, there's a possibility it wouldn't be banned. Being colorless allows it access to every instant and sorcery in the game, and being able to couple with every powerful effect in the game is somewhat problematic (and with more releases, the pool of powerful effects continues to grow). That being said, it makes every effect powerful--not just the powerful ones. Getting a free
Cultivate every turn sounds fun, but it quickly becomes backbreaking when you suddenly never have any mana problems
and quite a bit more mana than anyone else. Simple draw spells suddenly push you way far ahead in card advantage. The most basic kill spells keep opponents from ever having creatures again. It's arguably a very fun situation to be in...but only for the person with Panoptic Mirror.
And finally, being an artifact makes it easily steal-able and recurrable. It centralizes the game to being about who has it longer, or get it out more quickly, or protects it better (or all of these). Which swings us back around to it being colorless--most decks will need to consider running it, because literally every deck can run it (even those without a whole lot of spells). So in conclusion, it's very likely to fall under "Creates Undesirable Game States" and "Problematic Casual Omnipresence."
Not everyone agrees with these points or may have different opinions about why it should or shouldn't remain banned, which is fine. It is a card that gets discussed often.