Consumers decided - and Google also decided to ban manufacturers from building phones with another OS or they loose access to Google Play store and Google apps on the lucrative Android phones they also manufacture: https://arstechnica.com/gadgets/2018/07/googles-iron-grip-on...
(the article is from 2013 but was updated in 2018)
So no manufacturer that already builds Android phones will make an alternative OS phone for you - and that's effectively all of the mobile phone manufacturers given Apple makes all of the iOS devices.
Why haven't all the manufacturers filed a counter suit or otherwise work to pressure Google and cause them grief? They are all suffering so they should all work to reduce that suffering no? Only thing I can think of is that some manufacturers are doing just fine in this arrangement so there is no motivation to upend the apple cart.
(the article is from 2013 but was updated in 2018)
So no manufacturer that already builds Android phones will make an alternative OS phone for you - and that's effectively all of the mobile phone manufacturers given Apple makes all of the iOS devices.