We use Handlebars.js because it's a solid, performant JavaScript templating system with reasonable tooling. We adopted it more than 5 years ago now, and haven't felt significant pain points with it. Further, it's been easy to add i18n support and other useful extensions over time.

We might migrate to another templating engine if we migrate to a JavaScript framework with a built-in templating engine (basically because you have to), but I think it'd take something like that to make us want to switch to something else.