Why devices interface to Android via the headset jack

It seems that more and more devices interface with Android phones and tablets via the headset jack. As examples here is a brand new geiger counter: http://www.kjell.com/sortiment/telefoni-kommunikation/mobiltelefon-tillbehor/gadgets/geiger-och-uv-matare/geigermatare-for-mobilen-p96153#ProductDetailedInformation

And here is the new credit card processing unit for iZettle (the older one uses Bluetooth). https://www.izettle.com/se/card-readers

I have used the IZettle bluetooth device with different devices and it just doesn't work on all devices, due to as far as i understand different bluetooth stacks.

I recently tried to interface to an Arduino with a USB library through an OTG . It worked on one phone but not on the other.

My guess is why this is happening is that the implementations for Bluetooth and USB support on Android are too buggy or at least diverging in standards.

We do know however that people have standards for music and sound quality, which the manufacturers must abide too or people won't use the product at all. So we are sometimes back to analog modem signals it seems.