When you control the scripting environment, and would be instantiating in scripting rather than in serialised markup, Custom Elements are one of the more superfluous features of the web platform: everything you can do with them, you can do every bit as effectively without them, at no meaningful cost or cost saving.
The point where things get a little more interesting is when you introduce Shadow DOM, because then you do have genuine capabilities that can’t be expressed in other ways (though whether you want to may be another matter).
But the real kicker, in my assessment and conclusion, is that both of these features landed after the migration from XUL to HTML began. (Not all that long after, and they have since been used variously, but the moving initiatives began before they were available.)