However, I don't think there is any particular reason.
More important than "Why is 21 the limit" is that there is a limit after all.
Correct, that is one of the innovations of bitcoin, as we know fiat has no limit at all, while in the case of gold the limit is the gold that is on our planet, and while there are estimates about that amount that is all what they are, bitcoin as far as I know is the first form of money with a hard cap, and when we take into account its many other characteristics then this makes it incredibly valuable and one of the best assets to hold for the long term.