The reason why the Mirai botnet is still at large is: Android.
From a business perspective nobody wants to pay the costly people that can do microcontroller programming. Frontend devs are dirt cheap, especially for something as simple as that interface displaying the bar charts.
From employee perspective it was my impression that EE developers tend to get lower salaries than web developers.
But it could be the case that building an android or web app for a simple UI would take less dev-months than an embedded app with similar functionality.
There is also an enormous amount of flexibility gained when, instead if designing and building your own single-purpose device, you just use a cheap, mass produced, off the shelf, general purpose device.
From a business perspective nobody wants to pay the costly people that can do microcontroller programming. Frontend devs are dirt cheap, especially for something as simple as that interface displaying the bar charts.