Presumably for a "false accusation" conviction one would have to prove that (1) the accusations are false, and (2) the accuser was aware that the accusations were false. Merely failing to prove accusations (such as when there is insufficient evidence) would be insufficient for a false accusation conviction, as neither of these requirements are satisfied.

Isn't this already covered under libel?

You're not talked about a false allegation, you're talking about an unproved allegation. Those are very different things.

It's a spectrum, and we're only talking about one of the extremes:

<---- false allegation beyond a reasonable doubt ---- | ---- false allegation ---- ||| ---- true allegation ---- | ---- true allegation beyond a reasonable doubt ---->

Falsifying a crime and not having enough evidence are very different things. It takes a lot of work or luck to prove that intent.