Totally agree about the mobile aspect. Initially, I focused on making the website mobile-first, but we received numerous requests for a mobile app.
Since I'm not well-versed in mobile development and we're self-funded with a tight budget, I opted to super optimize the website for mobile instead. I figured optimizing for free traffic through SEO would be a fantastic starting point. Surprisingly, I stumbled upon a ton of high-traffic keywords with little to no competition, which got me pretty excited.
I think the popularity comes from the really good mobile experience, that would be a top priority from a service like this.