why windows XP is not free?

After the  elimination Windows XP,Why Microsoft don't take the strategy of "free  Windows"?The fuction of the free windows are limited by advertisements,if you want to use the full edition of the Windows,you may pay fopr it.Maybe ,Microsoft think it has an  obvious effect on the private user but there are too fifficult to attract  ebterprise users and take amount of installed capacity.
After the annoucement of stopping sauspport Windows XP on April8,2014,Microsoft will not provide any update  or bug fixes for the program,it also means that the users who use windows XP will face the  security threats that they never meet before.
Different from Linux and Mac OS,Windows has an absolute advantage in the desktop oprating system.althgough there are too many private users using the  pirated  Windows,enterprise users who used  legal windows will bring benefit to the microsoft.If these members can upgrade their system FROM WINDOWS XP TO WINDOWS 8,IT is no dout thet there is another income to microsoft.