Disclaimer: I haven't played with this since 2016, so may be out of date. Having played with the Alexa and Google home. My take on why there are no hits is that it's very clunky to invoke apps, especially for one-off queries. "Hey, Alexa/Google, Ask to do " or some variation on that. Starting an app and then interacting with it works fine for some use cases but not all. It really needs to be a flow of install app for…
I think the real problem though is that people aren't good at remembering voice commands, either how to invoke the command or that the command even exists. Without visual cues it's easy to forget everything.
You're not going to have a runaway hit when it's predicated on everyone remembering that special thing to say. It's like trying to have a runaway hit that had to be based on a keyboard shortcut.
All of those services are great for the basics, e.g. How's the weather / set a timer / remind me in N minutes / order toilet paper ... Everything else is a bit tougher in my opinion.