Would you want to be the dev responsible for writing a version of your software for all those OS's?!
Thats what the whole argument is fundamentally about. MSs idea of standardization is a members only club. Think of how many OSes can run perfectly happily on PC hardware. Programming for multiple osses and platforms CAN be easy with the correct standards (BMAX, gtk, python, java (non MS), openGL etc. are all testament to this that it can be done if greed wasnt a factor) What hinders a universal development is closed standards (like directX for example) All video cards support it and need it to use them at full capacity, but which is the only OS that can use it properly?
Its a vicious circle.