For one thing it offers a far cleaner user experience than Git does.
As a irregular hg user, I frequently run into (or help others with) issues with the ui, like "why does `hg log require `-f` to do something useful", "how do I refer to the current head commit (protip: it's not `tip`)", "how do I see changes relative to a branch (bookmark) on a remote" and "which set of commands am I supposed to use for history rewriting nowadays and how do they work?".
hg has some nice features, but the idea that it's got a fully intuitive and uniformly well designed interface is, in my experience, a total myth. Which isn't to say that git has a great CLI of course, just that the extent to which hg is better in this area is often overstated.
Which is that the difference between the CLIs in the early days were stark contrasts of good and bad, but now they both seem about equally mediocre, but in different ways, when directly contrasted. (Especially with the latest git release finally splitting checkout into switch and restore, they are really on increasingly similar footing.)