why is – in most fonts – the backslash more slanted than the "regular" slash? Is there any reason?
That is not something I have ever observed. Can you post an example?
Maybe because it's a technical character that's only really used in computer programming? So it doesn't need to fit in with running text as much? There are different versions of the forward slash, as well — the virgule is probably what you mean by a "regular" slash; there's also the solidus, which is used as a fraction bar.
If anything I would expect to see the backslash to be tilted less than the forward slash, because of the "f" and "j".
In any case, it seems to me there's little reason to make the slants symmetrical.
Somewhat to my surprise, the slash and backslash are not treated by Unicode as mirroring characters for bidirectional layout purposes. If they were, then that would be a strongish reason to make them symmetrical. But even though they're not, my inclination would be to make them symmetrical because in my experience any character with a flipped version is likely to be used as /mirrored punctuation\ by someone out there.