Neither of those libraries provide cross-shell completions, or coloured output, or "did you mean" suggestions, or even just command aliases, all of which I would consider basic features in a modern CLI. So you need to invest more time to provide those features, whereas they just exist in clap.
That's not to say that clap is always better, but it is significantly more full-featured than the alternatives, and for a larger project, those features are likely to be important.
For a smaller project, like something you're just making for yourself, I can see why you'd go for a less full-featured option, but given there's not much difference between clap and, say, argh that I feel like I'd get much benefit out of argh. If you're really looking for something simple, just use lexopt or something like that, and write the help text by hand.
