I read a comment from Richard Garfield in the days of "Revised edition" and he said that one of the flaws of Black Lotus was that there was no deck that could not be made better with the addition of Black Lotus.

It's an auto-include. A no-brainer. Everybody wants one. Every deck benefits from having it, and that's why it defeats the spirit of the game and should be left out of future editions.

So, you could say that it's because it's so powerful that it's an auto-include, but then that leads to a discussion about cards that are powerful on their own and what cards are enablers.