Microsoft spent a lot of money and resources trying to compete and failed.
Android/Apple just started early enough and by 2012 it was too late as most consumers have decided. To enter this market you have to be truly unique or else you are just copying the competitor and why would users switch if they are happy enough?
Is that really a monopoly if you had a third competitor come in and try?
(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.