/u/DerisionConsulting@lemmy.ca is right on the money. Mana paces the game, so anything that can break that is super good. In an otherwise even matchup, if one player has a Lotus while the other doesn’t, that can easily make the game. It’s not going to win the game in and of itself, but it’s a huge enabler to play the thing that will win you the game, before your opponent can reasonably do anything about it.
On top of that, it’s literally good in all decks. It’s been banned in every format besides Vintage, where it’s restricted to one (and not including casual/fan formats). It had to be banned partly for power reasons, but also because it makes deck-building less diverse. There’s no deck that wouldn’t want a Lotus if it could have one, much less four.
It’s also part of the Reserved List. After WotC overprinted cards, they essentially promised not to reprint certain ones. I think it’s a dumb decision, but they’ve annoyingly stuck to it (and players are worse off for it). Black Lotus is on that list. And it was alreadly limited in printings, because it was a rare card, and a bit of a design mistake.
It’s also simply an iconic card. Despite being a design mistake, it’s a major part of Magic history, and gets referenced all the time. To some extent, it’s famous for being famous. That makes it the biggest prize for collectors.
So, all this together, it has an incredibly high demand, a very limited supply, and no indication of a reprint anytime soon.
So I printed off a proxy at a professional card printer for 30¢. :)
I’ve made a habit of saying “Look, [city] was a powderkeg ready to go off before we even got there.” It’s come up in multiple campaigns.